Doped elongated semiconductors, growing such semiconductors, devices including such semiconductors and fabricating such devices

ABSTRACT

A bulk-doped semiconductor that is at least one of the following: a single crystal, an elongated and bulk-doped semiconductor that, at any point along its longitudinal axis, has a largest cross-sectional dimension less than 500 nanometers, and a free-standing and bulk-doped semiconductor with at least one portion having a smallest width of less than 500 nanometers. Such a semiconductor may comprise an interior core comprising a first semiconductor; and an exterior shell comprising a different material than the first semiconductor. Such a semiconductor may be elongated and may have, at any point along a longitudinal section of such a semiconductor, a ratio of the length of the section to a longest width is greater than 4:1, or greater than 10:1, or greater than 100:1, or even greater than 1000:1. At least one portion of such a semiconductor may a smallest width of less than 200 nanometers, or less than 150 nanometers, or less than 100 nanometers, or less than 80 nanometers, or less than 70 nanometers, or less than 60 nanometers, or less than 40 nanometers, or less than 20 nanometers, or less than 10 nanometers, or even less than 5 nanometers. Such a semiconductor may be a single crystal and may be free-standing. Such a semiconductor may be either lightly n-doped, heavily n-doped, lightly p-doped or heavily p-doped. Such a semiconductor may be doped during growth. Such a semiconductor may be part of a device, which may include any of a variety of devices and combinations thereof, and a variety of assembling techniques may be used to fabricate devices from such a semiconductor. Two or more of such a semiconductors, including an array of such semiconductors, may be combined to form devices, for example, to form a crossed p-n junction of a device. Such devices at certain sizes may exhibit quantum confinement and other quantum phenomena, and the wavelength of light emitted from one or more of such semiconductors may be controlled by selecting a width of such semiconductors. Such semiconductors and device made therefrom may be used for a variety of applications.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/935,776, entitled “Doped Elongated Semiconductors, Growing SuchSemiconductors, Devices Including Such Semiconductors And FabricatingSuch Devices, filed Aug. 22, 2001; which claims priority under 35 U.S.C.§119(e) to commonly-owned, co-pending U.S. Provisional PatentApplication Ser. No. 60/226,835, entitled, “Semiconductor Nanowires”,filed Aug. 22, 2000; Ser. No. 60/292,121, entitled, “SemiconductorNanowires”, filed May 18, 2001; Ser. No. 60/254,745, entitled, “Nanowireand Nanotube Nanosensors,” filed Dec. 11, 2000; Ser. No. 60/292,035,entitled “Nanowire and Nanotube Nanosensors,” filed May 18, 2001; Ser.No. 60/292,045, entitled “Nanowire Electronic Devices Including Memoryand Switching Devices,” filed May 18, 2001; and Ser. No. 60/291,896,entitled “Nanowire Devices Including Emissive Elements and Sensors,”filed May 18, 2001, each of which is hereby incorporated by reference inits entirety.

FIELD OF THE INVENTION

The present invention relates generally to sub-microelectronicsemiconductor devices, and more particularly to nanometer-scalesemiconductor articles, for example, nanowires, doped to provide n-typeand p-type conductivity, the growth of such articles, and thearrangement of such articles to fabricate devices.

BACKGROUND

Small-scale electronic technology relies to a large extent on doping ofvarious materials. Doping of semiconductor materials to form n-type andp-type semiconductor regions for making a variety of devices such asfield effect transistors, bipolar transistors, complementary inverters,tunnel diodes, and the like are well known.

Typical state-of-the-art semiconductor fabrication facilities involverelatively high cost, and require a clean room and the use of toxicchemicals such as hydrogen fluoride. While semiconductor andmicrofabrication technology is well-developed, there is a continuingneed for improvements, preferably including smaller-scale,environmentally-friendly fabrication, at lower cost.

SUMMARY

In an embodiment, provided is a free-standing bulk-doped semiconductorcomprising at least one portion having a smallest width of less than 500nanometers.

In another aspect of this embodiment, the semiconductor comprises: aninterior core comprising a first semiconductor; and an exterior shellcomprising a different material than the first semiconductor.

In another aspect of this embodiment, the semiconductor is elongated. Invarious optional features of this aspect, at any point along alongitudinal section of the semiconductor, a ratio of the length of thesection to a longest width is greater than 4:1, or greater than 10:1, orgreater than 100:1, or even greater than 1000:1.

In various aspects of this embodiment, at least one portion of thesemiconductor has a smallest width of less than 200 nanometers, or lessthan 150 nanometers, or less than 100 nanometers, or less than 80nanometers, or less than 70 nanometers, or less than 60 nanometers, orless than 40 nanometers, or less than 20 nanometers, or less than 10nanometers, or even less than 5 nanometers.

In various aspects of this embodiment, the semiconductor comprises asemiconductor from a group consisting of: Si, Ge, Sn, Se, Te, B,Diamond, P, B—C, B—P(BP₆), B—Si, Si—C, Si—Ge, Si—Sn and Ge—Sn, SiC,BN/BP/BAs, AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,BN/BP/BAs, AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,ZnO/ZnS/ZnSe/ZnTe, CdS/CdSe/CdTe, HgS/HgSe/HgTe, BeS/BeSe/BeTe/MgS/MgSe,GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr,CuI, AgF, AgCl, AgBr, AgI, BeSiN₂, CaCN₂, ZnGeP₂, CdSnAs₂, ZnSnSb₂,CuGeP₃, CuSi₂P₃, (Cu, Ag)(Al, Ga, In, Tl, Fe)(S, Se, Te)₂, Si₃N₄, Ge₃N₄,Al₂O₃, (Al, Ga, In)₂(S, Se, Te)₃, Al₂CO, and an appropriate combinationof two or more such semiconductors.

In various aspects of this embodiment, the semiconductor comprises adopant from a group consisting of: a p-type dopant from Group III of theperiodic table; an n-type dopant from Group V of the periodic table; ap-type dopant selected from a group consisting of: B, Al and In; ann-type dopant selected from a group consisting of: P, As and Sb; ap-type dopant from Group II of the periodic table; a p-type dopantselected from a group consisting of: Mg, Zn, Cd and Hg; a p-type dopantfrom Group IV of the periodic table; a p-type dopant selected from agroup consisting of: C and Si; or an n-type selected from a groupconsisting of: Si, Ge, Sn, S, Se and Te.

In another aspect of this embodiment, the semiconductor is part of adevice. In another aspect of this embodiment, the semiconductor isn-doped. In various optional features of this aspect, the semiconductoris either lightly n-doped or heavily n-doped.

In yet another aspect of this embodiment, the semiconductor is p-doped.In various optional features embodiments of this aspect, thesemiconductor is either lightly p-doped or heavily p-doped.

In another aspect of this embodiment, the semiconductor is a singlecrystal.

In additional various aspects of this embodiment, the semiconductor ismagnetic; the semiconductor comprises a dopant making the semiconductormagnetic the semiconductor is ferromagnetic; the semiconductor comprisesa dopant that makes the semiconductor ferromagnetic; and/or thesemiconductor comprises manganese.

In another embodiment, provided is an elongated and bulk-dopedsemiconductor that, at any point along its longitudinal axis, has alargest cross-sectional dimension less than 500 nanometers.

In an aspect of this embodiment, the semiconductor is free-standing.

In another aspect of this embodiment, the semiconductor comprises: aninterior core comprising a first semiconductor; and an exterior shellcomprising a different material than the first semiconductor.

In various aspects of this embodiment, at any point along thelongitudinal axis of the semiconductor, a ratio of the length of thesection to a longest width is greater than 4:1, or greater than 10:1, orgreater than 100:1 or even greater than 1000:1.

In various aspects of this embodiment, at least one longitudinal sectionof the semiconductor has a smallest width of less than 200 nanometers,or less than 150 nanometers, or less than 100 nanometers, or less than80 nanometers, or less than 70 nanometers, or less than 60 nanometers,or less than 40 nanometers, or less than 20 nanometers, or less than 10nanometers, or even less than 5 nanometers.

In various aspects of this embodiment, at least one longitudinal sectionof the semiconductor has a largest width of less than 200 nanometers, orless than 150 nanometers, or less than 100 nanometers, or less than 80nanometers, or less than 70 nanometers, or less than 60 nanometers, orless than 40 nanometers, or less than 20 nanometers, or less than 10nanometers, or even less than 5 nanometers.

In various aspects of this embodiment, the semiconductor comprises asemiconductor from a group consisting of: Si, Ge, Sn, Se, Te, B,Diamond, P, B—C, B—P(BP₆), B—Si, Si—C, Si—Ge, Si—Sn and Ge—Sn, SiC,BN/BP/BAs, AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,BN/BP/BAs, AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,ZnO/ZnS/ZnSe/ZnTe, CdS/CdSe/CdTe, HgS/HgSe/HgTe, BeS/BeSe/BeTe/MgS/MgSe,GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr,CuI, AgF, AgCl, AgBr, AgI, BeSiN₂, CaCN₂, ZnGeP₂, CdSnAs₂, ZnSnSb₂,CuGeP₃, CuSi₂P₃, (Cu, Ag)(Al, Ga, In, Tl, Fe)(S, Se, Te)₂, Si₃N₄, Ge₃N₄,Al₂O₃, (Al, Ga, In)₂(S, Se, Te)₃, Al₂CO, and an appropriate combinationof two or more such semiconductors.

In various aspects of this embodiment, the semiconductor comprises adopant from a group consisting of: a p-type dopant from Group III of theperiodic table; an n-type dopant from Group V of the periodic table; ap-type dopant selected from a group consisting of: B, Al and In; ann-type dopant selected from a group consisting of: P, As and Sb; ap-type dopant from Group II of the periodic table; a p-type dopantselected from a group consisting of: Mg, Zn, Cd and Hg; a p-type dopantfrom Group IV of the periodic table; a p-type dopant selected from agroup consisting of: C and Si; or an n-type dopant selected from a groupconsisting of: Si, Ge, Sn, S, Se and Te.

In another aspect of this embodiment, the semiconductor is part of adevice.

In another aspect of this embodiment, the semiconductor is n-doped. Invarious optional features of this aspect, the semiconductor is eitherlightly n-doped or heavily n- doped.

In yet another aspect of this embodiment, the semiconductor is p-doped.In various optional features embodiments of this aspect, thesemiconductor is either lightly p-doped or heavily p-doped.

In another aspect of this embodiment, the semiconductor is a singlecrystal.

In another embodiment, provided is a doped semiconductor comprising asingle crystal.

In an aspect of this embodiment, the semiconductor is bulk-doped.

In an aspect of this embodiment, the semiconductor is elongated. Invarious optional features of this aspect, at any point along alongitudinal section of the semiconductor, a ratio of the length of thesection to a longest width is greater than 4:1, or greater than 10:1, orgreater than 100:1, or even greater than 1000:1.

In various aspects of this embodiment, at least one portion of thesemiconductor has a smallest width of less than 200 nanometers, or lessthan 150 nanometers, or less than 100 nanometers, or less than 80nanometers, or less than 70 nanometers, or less than 60 nanometers, orless than 40 nanometers, or less than 20 nanometers, or less than 10nanometers, or even less than 5 nanometers.

In various aspects of this embodiment, the semiconductor comprises asemiconductor from a group consisting of: Si, Ge, Sn, Se, Te, B,Diamond, P, B—C, B—P(BP₆), B—Si, Si—C, Si—Ge, Si—Sn and Ge—Sn, SiC,BN/BP/BAs, AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,BN/BP/BAs, AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,ZnO/ZnS/ZnSe/ZnTe, CdS/CdSe/CdTe, HgS/HgSe/HgTe, BeS/BeSe/BeTe/MgS/MgSe,GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr,CuI, AgF, AgCl, AgBr, AgI, BeSiN₂, CaCN₂, ZnGeP₂, CdSnAs₂, ZnSnSb₂,CuGeP₃, CuSi₂P₃, (Cu, Ag)(Al, Ga, In, Tl, Fe)(S, Se, Te)₂, Si₃N₄, Ge₃N₄,Al₂O₃, (Al, Ga, In)₂(S, Se, Te)₃, Al₂CO, and an appropriate combinationof two or more such semiconductors.

In various aspects of this embodiment, the semiconductor comprises adopant from a group consisting of: a p-type dopant from Group III of theperiodic table; an n-type dopant from Group V of the periodic table; ap-type dopant selected from a group consisting of: B, Al and In; ann-type dopant selected from a group consisting of: P, As and Sb; ap-type dopant from Group II of the periodic table; a p-type dopantselected from a group consisting of: Mg, Zn, Cd and Hg; a p-type dopantfrom Group IV of the periodic table; a p-type dopant selected from agroup consisting of: C and Si; or an n-type dopant selected from a groupconsisting of: Si, Ge, Sn, S, Se and Te.

In yet another embodiment, provided is a doped semiconductor that is atleast one of the following: a single crystal, an elongated andbulk-doped semiconductor that, at any point along its longitudinal axis,has a largest cross-sectional dimension less than 500 nanometers, and afree-standing and bulk-doped semiconductor with at least one portionhaving a smallest width of less than 500 nanometers, where a phenomenaproduced by a section of the bulk-doped semiconductor exhibits a quantumconfinement caused by a dimension of the section.

In another aspect of this embodiment, the longitudinal section iscapable of emitting light in response to excitation, wherein awavelength of the emitted light is related to the width. In optionalfeatures of this aspect: the wavelength of the emitted light is afunction of the width; the longitudinal section is capable oftransporting electrical carriers without scattering; the longitudinalsection is capable of transporting electrical carriers such that theelectrical carriers pass through the longitudinal section ballistically;the longitudinal section is capable of transporting electrical carrierssuch that the electrical carriers pass through the longitudinal sectioncoherently; the longitudinal section is capable of transportingelectrical carriers such that the electrical carriers arespin-polarized; and/or the longitudinal section is capable oftransporting electrical carriers such that the spin-polarized electricalcarriers pass through the longitudinal section without losing spininformation.

In another embodiment, provided is a solution comprising one or moredoped semiconductors, wherein at least one of the semiconductors is atleast one of the following: a single crystal, an elongated andbulk-doped semiconductor that, at any point along its longitudinal axis,has a largest cross-sectional dimension less than 500 nanometers, and afree-standing and bulk-doped semiconductor with at least one portionhaving a smallest width of less than 500 nanometers.

In an aspect of this embodiment, the at least one semiconductor iselongated. In various optional features of this aspect, at any pointalong a longitudinal section of the semiconductor, a ratio of the lengthof the section to a longest width is greater than 4:1, or greater than10:1, or greater than 100:1 or even greater than 1000:1.

In various aspects of this embodiment, at least one portion of the atleast one semiconductor has a smallest width of less than 200nanometers, or less than 150 nanometers, or less than 100 nanometers, orless than 80 nanometers, or less than 70 nanometers, or less than 60nanometers, or less than 40 nanometers, or less than 20 nanometers, orless than 10 nanometers, or even less than 5 nanometers.

In various aspects of this embodiment, the at least one semiconductorcomprises a semiconductor from a group consisting of: Si, Ge, Sn, Se,Te, B, Diamond, P, B—C, B—P(BP₆), B—Si, Si—C, Si—Ge, Si—Sn and Ge—Sn,SiC, BN/BP/BAs, AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,BN/BP/BAs, AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,ZnO/ZnS/ZnSe/ZnTe, CdS/CdSe/CdTe, HgS/HgSe/HgTe, BeS/BeSe/BeTe/MgS/MgSe,GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr,CuI, AgF, AgCl, AgBr, AgI, BeSiN₂, CaCN₂, ZnGeP₂, CdSnAs₂, ZnSnSb₂,CuGeP₃, CuSi₂P₃, (Cu, Ag)(Al, Ga, In, Tl, Fe)(S, Se, Te)₂, Si₃N₄, Ge₃N₄,Al₂O₃, (Al, Ga, In)₂(S, Se, Te)₃, Al₂CO, and an appropriate combinationof two or more such semiconductors.

In various aspects of this embodiment, the at least one semiconductorcomprises a dopant from a group consisting of: a p-type dopant fromGroup III of the periodic table; an n-type dopant from Group V of theperiodic table; a p-type dopant selected from a group consisting of: B,Al and In; an n-type dopant selected from a group consisting of: P, Asand Sb; a p-type dopant from Group II of the periodic table; a p-typedopant selected from a group consisting of: Mg, Zn, Cd and Hg; a p-typedopant from Group IV of the periodic table; a p-type dopant selectedfrom a group consisting of: C and Si; or an n-type selected from a groupconsisting of: Si, Ge, Sn, S, Se and Te.

In another embodiment, provided is a device comprising one or more dopedsemiconductors, wherein at least one of the semiconductors is at leastone of the following: a single crystal, an elongated and bulk-dopedsemiconductor that, at any point along its longitudinal axis, has alargest cross-sectional dimension less than 500 nanometers, and afree-standing and bulk-doped semiconductor with at least one portionhaving a smallest width of less than 500 nanometers.

In an aspect of this embodiment, the device comprises at least two dopedsemiconductors, wherein both of the at least two doped semiconductors isat least one of the following: a single crystal, an elongated andbulk-doped semiconductor that, at any point along its longitudinal axis,has a largest cross-sectional dimension less than 500 nanometers, and afree-standing and bulk-doped semiconductor with at least one portionhaving a smallest width of less than 500 nanometers, and wherein a firstof the at least two doped semiconductors exhibits quantum confinementand a second of the at least two doped semiconductor manipulates thequantum confinement of the first.

In another aspect of this embodiment, the device comprises at least twodoped semiconductor, wherein both of the at least two dopedsemiconductors is at least one of the following: a single crystal, anelongated and bulk-doped semiconductor that, at any point along itslongitudinal axis, has a largest cross-sectional dimension less than 500nanometers, and a free-standing and bulk-doped semiconductor with atleast one portion having a smallest width of less than 500 nanometers.In various optional features of this embodiment: the at least twobulk-doped semiconductors are in physical contact with each other; afirst of the at least two bulk-doped semiconductors is of a firstconductivity type, and a second of the at least two bulk-dopedsemiconductors is of a second conductivity type; the first conductivitytype is n-type, and the second type of conductivity type is p- type;and/or the at least two bulk-doped semiconductors form a p-n junction.

In various aspects of this embodiment, the device comprises one or moreof the following: a switch; a diode; a Light-Emitting Diode; a tunneldiode; a Schottky diode; a Bipolar Junction Transistor; a Field EffectTransistor; an inverter; a complimentary inverter; an optical sensor; asensor for an analyte (e.g., DNA); a memory device; a dynamic memorydevice; a static memory device; a laser; a logic gate; an AND gate; aNAND gate; an EXCLUSIVE-AND gate; an OR gate; a NOR gate; anEXCLUSIVE-OR gate; a latch; a register; clock circuitry; a logic array;a state machine; a programmable circuit; an amplifier; a transformer; asignal processor; a digital circuit; an analog circuit; a light emissionsource; a photoluminescent device; an electroluminescent device; arectifier; a photodiode; a p-n solar cell.; a phototransistor; asingle-electron transistor; a single-photon emitter; a single-photondetector; a spintronic device; an ultra-sharp tip for atomic forcemicroscope; a scanning tunneling microscope; a field-emission device; aphotoluminescence tag; a photovoltaic device; a photonic band gapmaterials; a scanning near field optical microscope tips; and a circuitthat has digital and analog components.

In various aspects of this embodiment, for a device that includes one ormore of the device components listed in the previous paragraph, one ofthe device components may include the at least one semiconductor. In anoptional feature of this aspect, a plurality of the components of thedevice may include at least one semiconductor, where, for each devicecomponent, the at least one semiconductor is at least one of thefollowing: a single crystal, an elongated and bulk-doped semiconductorthat, at any point along its longitudinal axis, has a largestcross-sectional dimension less than 500 nanometers, and a free-standingand bulk-doped semiconductor with at least one portion having a smallestwidth of less than 500 nanometers.

In an aspect of this embodiment, the at least one semiconductor iselongated. In various optional features of this aspect, at any pointalong a longitudinal section of the semiconductor, a ratio of the lengthof the section to a longest width is greater than 4:1, or greater than10:1, or greater than 100:1 or even greater than 1000:1.

In various aspects of this embodiment, at least one portion of the atleast one semiconductor has a smallest width of less than 200nanometers, or less than 150 nanometers, or less than 100 nanometers, orless than 80 nanometers, or less than 70 nanometers, or less than 60nanometers, or less than 40 nanometers, or less than 20 nanometers, orless than 10 nanometers, or even less than 5 nanometers.

In various aspects of this embodiment, the at least one semiconductorcomprises a semiconductor from a group consisting of: Si, Ge, Sn, Se,Te, B, Diamond, P, B—C, B—P(BP₆), B—Si, Si—C, Si—Ge, Si—Sn and Ge—Sn,SiC, BN/BP/BAs, AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,BN/BP/BAs, AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,ZnO/ZnS/ZnSe/ZnTe, CdS/CdSe/CdTe, HgS/HgSe/HgTe, BeS/BeSe/BeTe/MgS/MgSe,GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr,CuI, AgF, AgCl, AgBr, AgI, BeSiN₂, CaCN₂, ZnGeP₂, CdSnAs₂, ZnSnSb₂,CuGeP₃, CuSi₂P₃, (Cu, Ag)(Al, Ga, In, Tl, Fe)(S, Se, Te)₂, Si₃N₄, Ge₃N₄,Al₂O₃, (Al, Ga, In)₂(S, Se, Te)₃, Al₂CO, and an appropriate combinationof two or more such semiconductors.

In various aspects of this embodiment, the at least one semiconductorcomprises a dopant from a group consisting of: a p-type dopant fromGroup III of the periodic table; an n-type dopant from Group V of theperiodic table; a p-type dopant selected from a group consisting of: B,Al and In; an n-type dopant selected from a group consisting of: P, Asand Sb; a p-type dopant from Group II of the periodic table; a p-typedopant selected from a group consisting of: Mg, Zn, Cd and Hg; a p-typedopant from Group IV of the periodic table; a p-type dopant selectedfrom a group consisting of: C and Si; or an n-type selected from a groupconsisting of: Si, Ge, Sn, S, Se and Te.

In another aspect of this embodiment, the device comprises anothersemiconductor that is electrically coupled to the at least onebulk-doped semiconductor.

In another aspect of this embodiment, the device comprises anothersemiconductor that is optically coupled to the at least one bulk-dopedsemiconductor. In yet another aspect of this embodiment, the devicecomprises another semiconductor that is magnetically coupled to the atleast one bulk-doped semiconductor.

In another aspect of this embodiment, the device comprises anothersemiconductor that physically contacts the at least one bulk-dopedsemiconductor.

In various aspects of this embodiment, the at least one semiconductor iscoupled to one or more of: an electrical contact; an optical contact; ora magnetic contact.

In another aspect of this embodiment, a conductivity of the at least onesemiconductor is controllable in response to a signal. In variousoptional features of this aspect: the conductivity of the at least onesemiconductor is controllable to have any value within a range ofvalues; the at least one semiconductor is switchable between two or morestates; the at least one semiconductor is switchable between aconducting state and an insulating state by the signal; two or morestates of the at least one semiconductor are maintainable without anapplied signal; the conductivity of the at least one semiconductor iscontrollable in response to an electrical signal; the conductivity ofthe at least one semiconductor is controllable in response to an opticalsignal; the conductivity of the at least one semiconductor iscontrollable in response to a magnetic signal; and/or the conductivityof the at least one semiconductor is controllable in response to asignal of a gate terminal.

In another aspect of this embodiment, at least two of the semiconductorsare arranged in an array, and at least one of the semiconductorsarranged in the array is a bulk-doped semiconductor comprising at leastone portion having a smallest width of less than 500 nanometers. In anoptional feature of this aspect, the array is an ordered array. Inanother optional feature of this embodiment, the array is not an orderedarray.

In yet another aspect of this embodiment, the device comprises two ormore separate and interconnected circuits, at least one of the circuitsnot comprising a bulk-doped semiconductor that comprises at least oneportion having a smallest width of less than 500 nanometers.

In another aspect of this embodiment, the device is embodied on a chiphaving one or more pinouts. In an optional feature of this embodiment,the chip comprises separate and interconnected circuits, at least one ofthe circuits not comprising a bulk-doped semiconductor that comprises atleast one portion having a smallest width of less than 500 nanometers.

In another embodiment, provided is a collection of reagents for growinga bulk-doped semiconductor that comprises at least one portion having asmallest width of less than 500 nanometers, the collection comprising asemiconductor reagent and a dopant reagent.

In an aspect of this embodiment, the at least one semiconductor iselongated. In various optional features of this aspect, at any pointalong a longitudinal section of the semiconductor, a ratio of the lengthof the section to a longest width is greater than 4:1, or greater than10:1, or greater than 100:1 or even greater than 1000:1.

In various aspects of this embodiment, at least one portion of the atleast one semiconductor has a smallest width of less than 200nanometers, or less than 150 nanometers, or less than 100 nanometers, orless than 80 nanometers, or less than 70 nanometers, or less than 60nanometers, or less than 40 nanometers, or less than 20 nanometers, orless than 10 nanometers, or even less than 5 nanometers.

In various aspects of this embodiment, the at least one semiconductorcomprises a semiconductor from a group consisting of: Si, Ge, Sn, Se,Te, B, Diamond, P, B—C, B—P(BP₆), B—Si, Si—C, Si—Ge, Si—Sn and Ge—Sn,SiC, BN/BP/BAs, AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,BN/BP/BAs, AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,ZnO/ZnS/ZnSe/ZnTe, CdS/CdSe/CdTe, HgS/HgSe/HgTe, BeS/BeSe/BeTe/MgS/MgSe,GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr,CuI, AgF, AgCl, AgBr, AgI, BeSiN₂, CaCN₂, ZnGeP₂, CdSnAs₂, ZnSnSb₂,CuGeP₃, CuSi₂P₃, (Cu, Ag)(Al, Ga, In, Tl, Fe)(S, Se, Te)₂, Si₃N₄, Ge₃N₄,Al₂O₃, (Al, Ga, In)₂(S, Se, Te)₃, Al₂CO, and an appropriate combinationof two or more such semiconductors.

In various aspects of this embodiment, the at least one semiconductorcomprises a dopant from a group consisting of: a p-type dopant fromGroup III of the periodic table; an n-type dopant from Group V of theperiodic table; a p-type dopant selected from a group consisting of: B,Al and In; an n-type dopant selected from a group consisting of: P, Asand Sb; a p-type dopant from Group II of the periodic table; a p-typedopant selected from a group consisting of: Mg, Zn, Cd and Hg; a p-typedopant from Group IV of the periodic table; a p-type dopant selectedfrom a group consisting of: C and Si; or an n-type selected from a groupconsisting of: Si, Ge, Sn, S, Se and Te.

In another embodiment, a semiconductor is doped during growth of thesemiconductor.

In various aspects of this embodiment: the semiconductor isfree-standing; the semiconductor has a smallest width of no more than100 nanometers; an extent of the doping is controlled; the dopedsemiconductor is grown by applying energy to a collection of molecules,the collection of molecules comprising molecules of the semiconductorand molecules of a dopant; an extent of the doping is controlled; aratio of an amount of the semiconductor molecules to an amount of thedopant molecules is controlled; the molecules are vaporized using alaser to form vaporized molecules; the semiconductor is grown from thevaporized molecules; the vaporized molecules are condensed into a liquidcluster; the semiconductor is grown from the liquid cluster; growing thesemiconductor is performed using laser-assisted catalytic growth; thecollection of molecules comprises a cluster of molecules of a catalystmaterial; a width of the semiconductor is controlled; and/or the widthof the semiconductor is controlled by controlling a width of thecatalyst cluster.

In additional aspects of this embodiment: the act of doping includesperforming chemical vapor deposition on at least the molecules; thegrown semiconductor has at least one portion having a smallest width ofless than 20 nanometers; the grown semiconductor has at least oneportion having a smallest width of less than 10 nanometers; and/or thegrown semiconductor has at least one portion having a smallest width ofless than 5 nanometers.

In yet other additional aspects of this embodiment: the grownsemiconductor is magnetic; the semiconductor is doped with a materialthat makes the grown semiconductor magnetic; the grown semiconductor isferromagnetic; the semiconductor is doped with a material that makes thegrown semiconductor ferromagnetic; the semiconductor is doped withmanganese.

In another aspect of this embodiment, the at least one semiconductor iselongated. In various optional features of this aspect, at any pointalong a longitudinal section of the semiconductor, a ratio of the lengthof the section to a longest width is greater than 4:1, or greater than10:1, or greater than 100:1 or even greater than 1000:1.

In various aspects of this embodiment, at least one portion of the atleast one semiconductor has a smallest width of less than 200nanometers, or less than 150 nanometers, or less than 100 nanometers, orless than 80 nanometers, or less than 70 nanometers, or less than 60nanometers, or less than 40 nanometers, or less than 20 nanometers, orless than 10 nanometers, or even less than 5 nanometers.

In various aspects of this embodiment, the at least one semiconductorcomprises a semiconductor from a group consisting of: Si, Ge, Sn, Se,Te, B, Diamond, P, B—C, B—P(BP₆), B—Si, Si—C, Si—Ge, Si—Sn and Ge—Sn,SiC, BN/BP/BAs, AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,BN/BP/BAs, AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,ZnO/ZnS/ZnSe/ZnTe, CdS/CdSe/CdTe, HgS/HgSe/HgTe, BeS/BeSe/BeTe/MgS/MgSe,GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr,CuI, AgF, AgCl, AgBr, AgI, BeSiN₂, CaCN₂, ZnGeP₂, CdSnAs₂, ZnSnSb₂,CuGeP₃, CuSi₂P₃, (Cu, Ag)(Al, Ga, In, Tl, Fe)(S, Se, Te)₂, Si₃N₄, Ge₃N₄,Al₂O₃, (Al, Ga, In)₂(S, Se, Te)₃, Al₂CO, and an appropriate combinationof two or more such semiconductors.

In various aspects of this embodiment, the at least one semiconductorcomprises a dopant from a group consisting of: a p-type dopant fromGroup III of the periodic table; an n-type dopant from Group V of theperiodic table; a p-type dopant selected from a group consisting of: B,Al and In; an n-type dopant selected from a group consisting of: P, Asand Sb; a p-type dopant from Group II of the periodic table; a p-typedopant selected from a group consisting of: Mg, Zn, Cd and Hg; a p-typedopant from Group IV of the periodic table; a p-type dopant selectedfrom a group consisting of: C and Si; or an n-type selected from a groupconsisting of: Si, Ge, Sn, S, Se and Te.

In another embodiment; a device is fabricated. One or moresemiconductors are contacted to a surface, where at least one of thesemiconductors is at least one of the following: a single crystal, anelongated and bulk-doped semiconductor that, at any point along itslongitudinal axis, has a largest cross-sectional dimension less than 500nanometers, and a free-standing and bulk-doped semiconductor with atleast one portion having a smallest width of less than 500 nanometers.In various aspects of this embodiment: the surface is a substrate; priorto contacting the surface, at least one of the semiconductors is grownby applying energy to molecules of a semiconductor and molecules of adopant; a solution is contacted comprising the one or moresemiconductors to the surface; one or more of the semiconductors arealigned on the surface using an electric field; an electric field isgenerated between at least two electrodes and one or more of thesemiconductors are positioned between the electrodes; another solutioncomprising one or more other semiconductors is contacted to the surface,where at least one of the other semiconductor is a bulk-dopedsemiconductor comprising at least one portion having a smallest width ofless than 500 nanometers; the surface is conditioned to attach the oneor more contacted semiconductors to the surface; forming channels on thesurface; patterns are formed on the surface; one or more of thesemiconductors are aligned on the surface using an electric field; theat least one semiconductor is elongated.

In various optional features of this aspect, at any point along alongitudinal section of the semiconductor, a ratio of the length of thesection to a longest width is greater than 4:1, or greater than 10:1, orgreater than 100:1 or even greater than 1000:1.

In various aspects of this embodiment, at least one portion of the atleast one semiconductor has a smallest width of less than 200nanometers, or less than 150 nanometers, or less than 100 nanometers, orless than 80 nanometers, or less than 70 nanometers, or less than 60nanometers, or less than 40 nanometers, or less than 20 nanometers, orless than 10 nanometers, or even less than 5 nanometers.

In various aspects of this embodiment, the at least one semiconductorcomprises a semiconductor from a group consisting of: Si, Ge, Sn, Se,Te, B, Diamond, P, B—C, B—P(BP₆), B—Si, Si—C, Si—Ge, Si—Sn and Ge—Sn,SiC, BN/BP/BAs, AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,BN/BP/BAs, AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,ZnO/ZnS/ZnSe/ZnTe, CdS/CdSe/CdTe, HgS/HgSe/HgTe, BeS/BeSe/BeTe/MgS/MgSe,GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr,CuI, AgF, AgCl, AgBr, AgI, BeSiN₂, CaCN₂, ZnGeP₂, CdSnAs₂, ZnSnSb₂,CuGeP₃, CuSi₂P₃, (Cu, Ag)(Al, Ga, In, Tl, Fe)(S, Se, Te)₂, Si₃N₄, Ge₃N₄,Al₂O₃, (Al, Ga, In)₂(S, Se, Te)₃, Al₂CO, and an appropriate combinationof two or more such semiconductors.

In various aspects of this embodiment, the at least one semiconductorcomprises a dopant from a group consisting of: a p-type dopant fromGroup III of the periodic table; an n-type dopant from Group V of theperiodic table; a p-type dopant selected from a group consisting of: B,Al and In; an n-type dopant selected from a group consisting of: P, Asand Sb; a p-type dopant from Group II of the periodic table; a p-typedopant selected from a group consisting of: Mg, Zn, Cd and Hg; a p-typedopant from Group IV of the periodic table; a p-type dopant selectedfrom a group consisting of: C and Si; or an n-type selected from a groupconsisting of: Si, Ge, Sn, S, Se and Te.

In another embodiment, generating light is generated by applying energyto one or more semiconductors causing the one or more semiconductors toemit light, wherein at least one of the semiconductors is at least oneof the following: a single crystal, an elongated and bulk-dopedsemiconductor that, at any point along its longitudinal axis, has alargest cross-sectional dimension less than 500 nanometers, and afree-standing and bulk-doped semiconductor with at least one portionhaving a smallest width of less than 500 nanometers.

In an aspect of this embodiment, the at least one semiconductor iselongated. In various optional features of this aspect, at any pointalong a longitudinal section of the semiconductor, a ratio of the lengthof the section to a longest width is greater than 4:1, or greater than10:1, or greater than 100:1 or even greater than 1000:1.

In various aspects of this embodiment, at least one portion of the atleast one semiconductor has a smallest width of less than 200nanometers, or less than 150 nanometers, or less than 100 nanometers, orless than 80 nanometers, or less than 70 nanometers, or less than 60nanometers, or less than 40 nanometers, or less than 20 nanometers, orless than 10 nanometers, or even less than 5 nanometers.

In various aspects of this embodiment, the at least one semiconductorcomprises a semiconductor from a group consisting of: Si, Ge, Sn, Se,Te, B, Diamond, P, B—C, B—P(BP₆), B—Si, Si—C, Si—Ge, Si—Sn and Ge—Sn,SiC, BN/BP/BAs, AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,BN/BP/BAs, AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,ZnO/ZnS/ZnSe/ZnTe, CdS/CdSe/CdTe, HgS/HgSe/HgTe, BeS/BeSe/BeTe/MgS/MgSe,GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr,CuI, AgF, AgCl, AgBr, AgI, BeSiN₂, CaCN₂, ZnGeP₂, CdSnAs₂, ZnSnSb₂,CuGeP₃, CuSi₂P₃, (Cu, Ag)(Al, Ga, In, Tl, Fe)(S, Se, Te)₂, Si₃N₄, Ge₃N₄,Al₂O₃, (Al, Ga, In)₂(S, Se, Te)₃, Al₂CO, and an appropriate combinationof two or more such semiconductors.

In various aspects of this embodiment, the at least one semiconductorcomprises a dopant from a group consisting of: a p-type dopant fromGroup III of the periodic table; an n-type dopant from Group V of theperiodic table; a p-type dopant selected from a group consisting of: B,Al and In; an n-type dopant selected from a group consisting of: P, Asand Sb; a p-type dopant from Group II of the periodic table; a p-typedopant selected from a group consisting of: Mg, Zn, Cd and Hg; a p-typedopant from Group IV of the periodic table; a p-type dopant selectedfrom a group consisting of: C and Si; or an n-type selected from a groupconsisting of: Si, Ge, Sn, S, Se and Te.

In various aspects of this embodiment: the at least one semiconductor isa bulk-doped; the semiconductor comprises a direct-band-gapsemiconductor; a voltage is applied across a junction of two crossedsemiconductors, each semiconductor having a smallest width of less than500 nanometers; each semiconductor has a smallest width of less than 100nanometers; a wavelength of the emitted light is controlled bycontrolling a dimension of the at least one semiconductor having asmallest width of less than 100 nanometers; the semiconductor iselongated, and a width of the elongated semiconductor is controlled; thesemiconductor has a property that a mass of the semiconductor emitslight at a first wavelength if the mass has a minimum shortestdimension, and the controlled dimension of the semiconductor is lessthan the minimum shortest dimension.

In another embodiment, a device having at least a doped semiconductorcomponent and one or more other components is fabricated. Asemiconductor is doped during its growth to produce the dopedsemiconductor component, and the doped semiconductor component isattached to at least one of the one or more other components.

In an aspect of this embodiment, the doped semiconductor is at least oneof the following: a single crystal, an elongated and bulk-dopedsemiconductor that, at any point along its longitudinal axis, has alargest cross-sectional dimension less than 500 nanometers, and afree-standing and bulk-doped semiconductor with at least one portionhaving a smallest width of less than 500 nanometers.

In various aspects of this embodiment: the semiconductor component is atleast part of a nanowire; the semiconductor is doped during growth ofthe semiconductor.

In another embodiment provided is a process for controllably assemblinga semiconductor device having elongated elements with a characteristicdimension in a transverse direction of the element on a nanometer scale,the process comprising: producing at least one first elements of a firstdoping type, orienting said first element in a first direction, andconnecting said first element to at least one first contact to allow anelectrical current to flow through the first element.

In various aspects of this embodiment: the process further comprisesproducing at least one second elements of a second doping type,orienting said second element in a second direction different from thefirst direction, enabling an electrical contact between the firstelement and the second element, and connecting said second element to atleast one second contact to allow an electrical current to flow betweenthe first and second element; the process further comprises connectingsaid first element to spaced-apart contacts and arranging a gateelectrode proximate to the first element between the spaced-apartcontacts, thereby forming an FET; the first doping type is one of n-typeor p-type; the second doping type is n-type if the first doping type isp-type, and p-type if the first doping type is n-type; the first elementis oriented by applying at least one of an electric field or a fluidflow; the first element is suspended in the fluid flow; the firstelement is oriented by applying a mechanical tool; the second element isoriented by applying at least one of an electric field or a fluid flow;the second element is suspended in the fluid flow; the second element isoriented by applying a mechanical tool.

In yet another embodiment, provided is a semiconductor device,comprising: a silicon substrate having an array of metal contacts; acrossbar switch element formed in electrical communication with thearray and having a first bar formed of a p-type semiconductor nanowire,and a second bar formed of an n-type semiconductor nanowire and beingspaced away from the first bar and being disposed transversely thereto.

In an aspect of this embodiment, the second bar is spaces between 1-10nm from the first bar.

In another embodiment, provided is a method for manufacturing a nanowiresemiconductor device comprising positioning a first nanowire between twocontact points by applying a potential between the contact points; andpositioning a second nanowire between two other contact points.

In another embodiment, provided is a method for manufacturing a nanowiresemiconductor device comprising forming a surface with one or moreregions that selectively attract nanowires.

In another embodiment, provided is a method for manufacturing alight-emitting diode from nanowires, the diode having an emissionwavelength determined by a dimension of a p-n junction between two dopednanowires.

In yet another embodiment, provided is a method for manufacturing asemiconductor junction by crossing a p-type nanowire and an n-typenanowire.

In another embodiment, provided is a method of assembling one or moreelongated structures on a surface, where the method comprising acts of:flowing a fluid that comprises the one or more elongated structures ontothe surface; and aligning the one or more elongated structures on thesurface to form an array of the elongated structures.

In various embodiments of this method: flowing comprises flowing thefluid in a first direction and aligning comprises aligning the one ormore elongated structures as the fluid flows in the first direction toform a first layer of arrayed structures, and the method furthercomprises changing a direction of the flow from the first direction to asecond direction, and repeating the acts of flowing and aligning; atleast a first elongated structure from the first layer contacts at leasta second elongated structure from the second array; one of the first andsecond elongated structures is doped semiconductor of a firstconductivity type and another of first and second elongated structuresis doped semiconductor of a second conductivity type; the firstconductivity type is p-type and the second conductivity type is n-type,and wherein the first and second elongated structures form a p-njunction; the surface is a surface of a substrate; the method furthercomprises transferring the array of elongated structures from thesurface of the substrate to a surface of another substrate; transferringcomprises stamping; the one or more elongated structured are alignedonto the surface while still comprised in the fluid; conditioning thesurface with one or more functionalities that attract the one or moreelongated structures to particular positions on the surface, and the actof aligning comprises attracting the one or more elongated structures tothe particular positions using the one or more functionalities; the actof conditioning comprises conditioning the surface with one or moremolecules; the act of conditioning comprises conditioning the surfacewith one or more charges; the act of conditioning comprises conditioningthe surface with one or more magnetos; the act of conditioning comprisesconditioning the surface with one or more light intensities;conditioning the surface with one or more functionalities that attractthe one or more elongated structures to particular positions on thesurface using chemical force; the act of conditioning comprisesconditioning the surface with one or more functionalities that attractthe one or more elongated structures to particular positions on thesurface using optical force; the act of conditioning comprisesconditioning the surface with one or more functionalities that attractthe one or more elongated structures to particular positions on thesurface using electrostatic force; the act of conditioning comprisesconditioning the surface with one or more functionalities that attractthe one or more elongated structures to particular positions on thesurface using magnetic force; the method further comprises patterningthe surface to receive the one or more elongated structures atparticular positions on the surface; the act of patterning comprisecreating physical patterns on the surface; the physical patterns aretrenches; the physical patterns are steps; the surface is a surface of asubstrate, and creating physical patterns on the surface comprises usingcrystal lattice steps of the substrate; the surface is a surface of asubstrate, and creating physical patterns on the surface comprises usingself-assembled di-block polymer strips; creating physical patterns onthe surface comprises using patterns; creating physical patterns on thesurface comprises using imprinted patterns; and/or the act of flowingcomprises controlling the flow of the fluid using a channel.

In additional aspects of this embodiment: at least one of the elongatedstructures are semiconductors; at least one of the elongated structuresare doped semiconductors; at least one of the elongated structures arebulk-doped semiconductors; at least one of the structures is a dopedsingle-crystal semiconductor; at least one of the structures is anelongated and bulk-doped semiconductor that, at any point along itslongitudinal axis, has a largest cross-sectional dimension less than 500nanometers; at least one of the structures is a free-standing andbulk-doped semiconductor with at least one portion having a smallestwidth of less than 500 nanometers; at least one of the structures is adoped semiconductor that is at least one of the following: a singlecrystal, an elongated and bulk-doped semiconductor that, at any pointalong its longitudinal axis, has a largest cross-sectional dimensionless than 500 nanometers, and a free-standing and bulk-dopedsemiconductor with at least one portion having a smallest width of lessthan 500 nanometers; the doped semiconductor comprises a semiconductorselected from a group consisting of: Si, Ge, Sn, Se, Te, B, Diamond, P,B—C, B—P(BP₆), B—Si, Si—C, Si—Ge, Si—Sn and Ge—Sn, SiC, BN/BP/BAs,AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb, BN/BP/BAs,AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,ZnO/ZnS/ZnSe/ZnTe, CdS/CdSe/CdTe, HgS/HgSe/HgTe, BeS/BeSe/BeTe/MgS/MgSe,GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr,CuI, AgF, AgCl, AgBr, AgI, BeSiN₂, CaCN₂, ZnGeP₂, CdSnAs₂, ZnSnSb₂,CuGeP₃, CuSi₂P₃, (Cu, Ag)(Al, Ga, In, Tl, Fe)(S, Se, Te)₂, Si₃N₄, Ge₃N₄,Al₂O₃, (Al, Ga, In)₂(S, Se, Te)₃, Al₂CO; the doped semiconductorcomprises a dopant selected from a group consisting of: a p-type dopantfrom Group III of the periodic table; an n-type dopant from Group V ofthe periodic table; a p-type dopant selected from a group consisting of:B, Al and In; an n-type dopant selected from a group consisting of: P,As and Sb; a p-type dopant from Group II of the periodic table; a p-typedopant selected from a group consisting of: Mg, Zn, Cd and Hg; a p-typedopant from Group IV of the periodic table; a p-type dopant selectedfrom a group consisting of: C and Si; and an n-type selected from agroup consisting of: Si, Ge, Sn, S, Se and Te; the doped semiconductoris doped during growth of the semiconductor.

In another embodiment, disclosed is a method of assembling one or moreelongated structures on a surface, wherein one or more of the elongatedstructures are at least one of the following: a single crystal, anelongated and bulk-doped semiconductor that, at any point along itslongitudinal axis, has a largest cross-sectional dimension less than 500nanometers, and a free-standing and bulk-doped semiconductor with atleast one portion having a smallest width of less than 500 nanometers,and wherein the method comprises acts of conditioning the surface withone or more functionalities that attract the one or more elongatedstructures to particular positions on the surface, and aligning the oneor more elongated structures by attracting the one or more elongatedstructures to the particular positions using the one or morefunctionalities.

In various aspects of this embodiment: the act of conditioning comprisesconditioning the surface with one or more molecules; the act ofconditioning comprises conditioning the surface with one or morecharges; the act of conditioning comprises conditioning the surface withone or more magnetos; the act of conditioning comprises conditioning thesurface with one or more light intensities; conditioning the surfacewith one or more functionalities that attract the one or more elongatedstructures to particular positions on the surface using chemical force;the act of conditioning comprises conditioning the surface with one ormore functionalities that attract the one or more elongated structuresto particular positions on the surface using optical force; the act ofconditioning comprises conditioning the surface with one or morefunctionalities that attract the one or more elongated structures toparticular positions on the surface using electrostatic force; and/orthe act of conditioning comprises conditioning the surface with one ormore functionalities that attract the one or more elongated structuresto particular positions on the surface using magnetic force.

In additional aspects of this embodiment: at least one of the elongatedstructures are semiconductors; at least one of the elongated structuresare doped semiconductors; at least one of the elongated structures arebulk-doped semiconductors; at least one of the structures is a dopedsingle-crystal semiconductor; at least one of the structures is anelongated and bulk-doped semiconductor that, at any point along itslongitudinal axis, has a largest cross-sectional dimension less than 500nanometers; at least one of the structures is a free-standing andbulk-doped semiconductor with at least one portion having a smallestwidth of less than 500 nanometers; at least one of the structures is adoped semiconductor that is at least one of the following: a singlecrystal, an elongated and bulk-doped semiconductor that, at any pointalong its longitudinal axis, has a largest cross-sectional dimensionless than 500 nanometers, and a free-standing and bulk-dopedsemiconductor with at least one portion having a smallest width of lessthan 500 nanometers; the doped semiconductor comprises a semiconductorselected from a group consisting of: Si, Ge, Sn, Se, Te, B, Diamond, P,B—C, B—P(BP₆), B—Si, Si—C, Si—Ge, Si—Sn and Ge—Sn, SiC, BN/BP/BAs,AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb, BN/BP/BAs,AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,ZnO/ZnS/ZnSe/ZnTe, CdS/CdSe/CdTe, HgS/HgSe/HgTe, BeS/BeSe/BeTe/MgS/MgSe,GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr,CuI, AgF, AgCl, AgBr, AgI, BeSiN₂, CaCN₂, ZnGeP₂, CdSnAs₂, ZnSnSb₂,CuGeP₃, CuSi₂P₃, (Cu, Ag)(Al, Ga, In, Tl, Fe)(S, Se, Te)₂, Si₃N₄, Ge₃N₄,AL₂O₃), (Al, Ga, In)₂(S, Se, Te)₃, Al₂CO; the doped semiconductorcomprises a dopant selected from a group consisting of: a p-type dopantfrom Group III of the periodic table; an n-type dopant from Group V ofthe periodic table; a p-type dopant selected from a group consisting of:B, Al and In; an n-type dopant selected from a group consisting of: P,As and Sb; a p-type dopant from Group II of the periodic table; a p-typedopant selected from a group consisting of: Mg, Zn, Cd and Hg; a p-typedopant from Group IV of the periodic table; a p-type dopant selectedfrom a group consisting of: C and Si; and an n-type selected from agroup consisting of: Si, Ge, Sn, S, Se and Te; the doped semiconductoris doped during growth of the semiconductor.

In another embodiment, disclosed is a method of assembling a pluralityof elongated structures on a surface, wherein one or more of theelongated structures are at least one of the following: a singlecrystal, an elongated and bulk-doped semiconductor that, at any pointalong its longitudinal axis, has a largest cross-sectional dimensionless than 500 nanometers, and a free-standing and bulk-dopedsemiconductor with at least one portion having a smallest width of lessthan 500 nanometers, and wherein the method comprises acts of:depositing the plurality of elongated structures onto the surface; andelectrically charging the surface to produce electrostatic forcesbetween two or more of the plurality of the elongated structures.

In various embodiments of this embodiment: the electrostatic forcescause the two or more elongated structures to align themselves; theelectrostatic forces cause the two or more elongated structures to alignthemselves into one or more patterns; and/or the one or more patternscomprise a parallel array.

In additional aspects of this embodiment: at least one of the elongatedstructures are semiconductors; at least one of the elongated structuresare doped semiconductors; at least one of the elongated structures arebulk-doped semiconductors; at least one of the structures is a dopedsingle-crystal semiconductor; at least one of the structures is anelongated and bulk-doped semiconductor that, at any point along itslongitudinal axis, has a largest cross-sectional dimension less than 500nanometers; at least one of the structures is a free-standing andbulk-doped semiconductor with at least one portion having a smallestwidth of less than 500 nanometers; at least one of the structures is adoped semiconductor that is at least one of the following: a singlecrystal, an elongated and bulk-doped semiconductor that, at any pointalong its longitudinal axis, has a largest cross-sectional dimensionless than 500 nanometers, and a free-standing and bulk-dopedsemiconductor with at least one portion having a smallest width of lessthan 500 nanometers; the doped semiconductor comprises a semiconductorselected from a group consisting of: Si, Ge, Sn, Se, Te, B, Diamond, P,B—C, B—P(BP₆), B—Si, Si—C, Si—Ge, Si—Sn and Ge—Sn, SiC, BN/BP/BAs,AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb, BN/BP/BAs,AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,ZnO/ZnS/ZnSe/ZnTe, CdS/CdSe/CdTe, HgS/HgSe/HgTe, BeS/BeSe/BeTe/MgS/MgSe,GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr,CuI, AgF, AgCl, AgBr, AgI, BeSiN₂, CaCN₂, ZnGeP₂, CdSnAs₂, ZnSnSb₂,CuGeP₃, CuSi₂P₃, (Cu, Ag)(Al, Ga, In, Tl, Fe)(S, Se, Te)₂, Si₃N₄, Ge₃N₄,Al₂O₃, (Al, Ga, In)₂(S, Se, Te)₃, Al₂CO; the doped semiconductorcomprises a dopant selected from a group consisting of: a p-type dopantfrom Group III of the periodic table; an n-type dopant from Group V ofthe periodic table; a p-type dopant selected from a group consisting of:B, Al and In; an n-type dopant selected from a group consisting of: P,As and Sb; a p-type dopant from Group II of the periodic table; a p-typedopant selected from a group consisting of: Mg, Zn, Cd and Hg; a p-typedopant from Group IV of the periodic table; a p-type dopant selectedfrom a group consisting of: C and Si; and an n-type selected from agroup consisting of: Si, Ge, Sn, S, Se and Te; the doped semiconductoris doped during growth of the semiconductor.

In yet another embodiment, provided is a method of assembling aplurality of elongated structures on a surface, wherein one or more ofthe elongated structures are at least one of the following: a singlecrystal, an elongated and bulk-doped semiconductor that, at any pointalong its longitudinal axis, has a largest cross-sectional dimensionless than 500 nanometers, and a free-standing and bulk-dopedsemiconductor with at least one portion having a smallest width of lessthan 500 nanometers, and wherein the method comprises acts of:dispersing the one or more elongated structures on a surface of a liquidphase to form a Langmuir-Blodgett film; compressing theLangmuir-Blodgett film; and transferring the compressedLangmuir-Blodgett film onto a surface.

In an aspect of this embodiment: the surface is the surface of asubstrate.

In additional aspects of this embodiment: at least one of the elongatedstructures are semiconductors; at least one of the elongated structuresare doped semiconductors; at least one of the elongated structures arebulk-doped semiconductors; at least one of the structures is a dopedsingle-crystal semiconductor; at least one of the structures is anelongated and bulk-doped semiconductor that, at any point along itslongitudinal axis, has a largest cross-sectional dimension less than 500nanometers; at least one of the structures is a free-standing andbulk-doped semiconductor with at least one portion having a smallestwidth of less than 500 nanometers; at least one of the structures is adoped semiconductor that is at least one of the following: a singlecrystal, an elongated and bulk-doped semiconductor that, at any pointalong its longitudinal axis, has a largest cross-sectional dimensionless than 500 nanometers, and a free-standing and bulk-dopedsemiconductor with at least one portion having a smallest width of lessthan 500 nanometers; the doped semiconductor comprises a semiconductorselected from a group consisting of: Si, Ge, Sn, Se, Te, B, Diamond, P,B—C, B—P(BP₆), B—Si, Si—C, Si—Ge, Si—Sn and Ge—Sn, SiC, BN/BP/BAs,AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb, BN/BP/BAs,AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,ZnO/ZnS/ZnSe/ZnTe, CdS/CdSe/CdTe, HgS/HgSe/HgTe, BeS/BeSe/BeTe/MgS/MgSe,GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr,CuI, AgF, AgCl, AgBr, AgI, BeSiN₂, CaCN₂, ZnGeP₂, CdSnAs₂, ZnSnSb₂,CuGeP₃, CuSi₂P₃, (Cu, Ag)(Al, Ga, In, Tl, Fe)(S, Se, Te)₂, Si₃N₄, Ge₃N₄,Al₂O₃, (Al, Ga, In)₂(S, Se, Te)₃, Al₂CO; the doped semiconductorcomprises a dopant selected from a group consisting of: a p-type dopantfrom Group III of the periodic table; an n-type dopant from Group V ofthe periodic table; a p-type dopant selected from a group consisting of:B, Al and In; an n-type dopant selected from a group consisting of: P,As and Sb; a p-type dopant from Group II of the periodic table; a p-typedopant selected from a group consisting of: Mg, Zn, Cd and Hg; a p-typedopant from Group IV of the periodic table; a p-type dopant selectedfrom a group consisting of: C and Si; and an n-type selected from agroup consisting of: Si, Ge, Sn, S, Se and Te; the doped semiconductoris doped during growth of the semiconductor.

In another embodiment, provided is a method of assembling a plurality ofone or more elongated structures on a surface, wherein at least one ofthe elongated structures are at least one of the following: a singlecrystal, an elongated and bulk-doped semiconductor that, at any pointalong its longitudinal axis, has a largest cross-sectional dimensionless than 500 nanometers, and a free-standing and bulk-dopedsemiconductor with at least one portion having a smallest width of lessthan 500 nanometers, and wherein the method comprises acts of:dispersing the one or more elongated structures in a flexible matrix;stretching the flexible matrix in a direction to produce a shear forceon the one or more elongated structures that causes the at least oneelongated structure to align in the direction; removing the flexiblematrix; and transferring the at least one aligned elongated structure toa surface.

In various aspects of this embodiment: the direction is parallel to aplane of the surface the act of stretching comprises stretching theflexible matrix with an electrically- induced force; the act ofstretching comprises stretching the flexible matrix with anoptically-induced force; the act of stretching comprises stretching theflexible matrix with a mechanically-induced force; the act of stretchingcomprises stretching the flexible matrix with a magnetically-inducedforce; the surface is a surface of a substrate; the flexible matrix is apolymer.

In additional aspects of this embodiment: at least one of the elongatedstructures are semiconductors; at least one of the elongated structuresare doped semiconductors; at least one of the elongated structures arebulk-doped semiconductors; at least one of the structures is a dopedsingle-crystal semiconductor; at least one of the structures is anelongated and bulk-doped semiconductor that, at any point along itslongitudinal axis, has a largest cross-sectional dimension less than 500nanometers; at least one of the structures is a free-standing andbulk-doped semiconductor with at least one portion having a smallestwidth of less than 500 nanometers; at least one of the structures is adoped semiconductor that is at least one of the following: a singlecrystal, an elongated and bulk-doped semiconductor that, at any pointalong its longitudinal axis, has a largest cross-sectional dimensionless than 500 nanometers, and a free-standing and bulk-dopedsemiconductor with at least one portion having a smallest width of lessthan 500 nanometers; the doped semiconductor comprises a semiconductorselected from a group consisting of: Si, Ge, Sn, Se, Te, B, Diamond, P,B—C, B—P(BP₆), B—Si, Si—C, Si—Ge, Si—Sn and Ge—Sn, SiC, BN/BP/BAs,AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb, BN/BP/BAs,AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,ZnO/ZnS/ZnSe/ZnTe, CdS/CdSe/CdTe, HgS/HgSe/HgTe, BeS/BeSe/BeTe/MgS/MgSe,GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr,CuI, AgF, AgCl, AgBr, AgI, BeSiN₂, CaCN₂, ZnGeP₂, CdSnAs₂, ZnSnSb₂,CuGeP₃, CuSi₂P₃, (Cu, Ag)(Al, Ga, In, Tl, Fe)(S, Se, Te)₂, Si₃N₄, Ge₃N₄,Al₂O₃, (Al, Ga, In)₂(S, Se, Te)₃, Al₂CO; the doped semiconductorcomprises a dopant selected from a group consisting of: a p-type dopantfrom Group III of the periodic table; an n-type dopant from Group V ofthe periodic table; a p-type dopant selected from a group consisting of:B, Al and In; an n-type dopant selected from a group consisting of: P,As and Sb; a p-type dopant from Group II of the periodic table; a p-typedopant selected from a group consisting of: Mg, Zn, Cd and Hg; a p-typedopant from Group IV of the periodic table; a p-type dopant selectedfrom a group consisting of: C and Si; and an n-type selected from agroup consisting of: Si, Ge, Sn, S, Se and Te; the doped semiconductoris doped during growth of the semiconductor.

In another embodiment, provided is a system for growing a dopedsemiconductor, the system comprising: means for providing a molecules ofthe semiconductor and molecules of a dopant; and means for doping themolecules of the semiconductor with the molecules of the dopant duringgrowth of the semiconductor to produce the doped semiconductor.

In additional aspects of this embodiment: at least one of the elongatedstructures are semiconductors; at least one of the elongated structuresare doped semiconductors; at least one of the elongated structures arebulk-doped semiconductors; at least one of the structures is a dopedsingle-crystal semiconductor; at least one of the structures is anelongated and bulk-doped semiconductor that, at any point along itslongitudinal axis, has a largest cross-sectional dimension less than 500nanometers; at least one of the structures is a free-standing andbulk-doped semiconductor with at least one portion having a smallestwidth of less than 500 nanometers; at least one of the structures is adoped semiconductor that is at least one of the following: a singlecrystal, an elongated and bulk-doped semiconductor that, at any pointalong its longitudinal axis, has a largest cross-sectional dimensionless than 500 nanometers, and a free-standing and bulk-dopedsemiconductor with at least one portion having a smallest width of lessthan 500 nanometers; the doped semiconductor comprises a semiconductorselected from a group consisting of: Si, Ge, Sn, Se, Te, B, Diamond, P,B—C, B—P(BP₆), B—Si, Si—C, Si—Ge, Si—Sn and Ge—Sn, SiC, BN/BP/BAs,AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb, BN/BP/BAs,AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,ZnO/ZnS/ZnSe/ZnTe, CdS/CdSe/CdTe, HgS/HgSe/HgTe, BeS/BeSe/BeTe/MgS/MgSe,GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr,CuI, AgF, AgCl, AgBr, AgI, BeSiN₂, CaCN₂, ZnGeP₂, CdSnAs₂, ZnSnSb₂,CuGeP₃, CuSi₂P₃, (Cu, Ag)(Al, Ga, In, Tl, Fe)(S, Se, Te)₂, Si₃N₄, Ge₃N₄,Al₂O₃, (Al, Ga, In)₂(S, Se, Te)₃, Al₂CO; the doped semiconductorcomprises a dopant selected from a group consisting of: a p-type dopantfrom Group III of the periodic table; an n-type dopant from Group V ofthe periodic table; a p-type dopant selected from a group consisting of:B, Al and In; an n-type dopant selected from a group consisting of: P,As and Sb; a p-type dopant from Group II of the periodic table; a p-typedopant selected from a group consisting of: Mg, Zn, Cd and Hg; a p-typedopant from Group IV of the periodic table; a p-type dopant selectedfrom a group consisting of: C and Si; and an n-type selected from agroup consisting of: Si, Ge, Sn, S, Se and Te; the doped semiconductoris doped during growth of the semiconductor.

In another embodiment, provided is a system for assembling one or moreelongated structures on a surface, the system comprising: means forflowing a fluid that comprises the one or more elongated structures ontothe surface; and means for aligning the one or more elongated structureson the surface to form an array of the elongated structures.

In additional aspects of this embodiment: at least one of the elongatedstructures are semiconductors; at least one of the elongated structuresare doped semiconductors; at least one of the elongated structures arebulk-doped semiconductors; at least one of the structures is a dopedsingle-crystal semiconductor; at least one of the structures is anelongated and bulk-doped semiconductor that, at any point along itslongitudinal axis, has a largest cross-sectional dimension less than 500nanometers; at least one of the structures is a free-standing andbulk-doped semiconductor with at least one portion having a smallestwidth of less than 500 nanometers; at least one of the structures is adoped semiconductor that is at least one of the following: a singlecrystal, an elongated and bulk-doped semiconductor that, at any pointalong its longitudinal axis, has a largest cross-sectional dimensionless than 500 nanometers, and a free-standing and bulk-dopedsemiconductor with at least one portion having a smallest width of lessthan 500 nanometers; the doped semiconductor comprises a semiconductorselected from a group consisting of: Si, Ge, Sn, Se, Te, B, Diamond, P,B—C, B—P(BP₆), B—Si, Si—C, Si—Ge, Si—Sn and Ge—Sn, SiC, BN/BP/BAs,AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb, BN/BP/BAs,AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,ZnO/ZnS/ZnSe/ZnTe, CdS/CdSe/CdTe, HgS/HgSe/HgTe, BeS/BeSe/BeTe/MgS/MgSe,GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr,CuI, AgF, AgCl, AgBr, AgI, BeSiN₂, CaCN₂, ZnGeP₂, CdSnAs₂, ZnSnSb₂,CuGeP₃, CuSi₂P₃, (Cu, Ag)(Al, Ga, In, Tl, Fe)(S, Se, Te)₂, Si₃N₄, Ge₃N₄,Al₂O₃, (Al, Ga, In)₂(S, Se, Te)₃, Al₂CO; the doped semiconductorcomprises a dopant selected from a group consisting of: a p-type dopantfrom Group III of the periodic table; an n-type dopant from Group V ofthe periodic table; a p-type dopant selected from a group consisting of:B, Al and In; an n-type dopant selected from a group consisting of: P,As and Sb; a p-type dopant from Group II of the periodic table; a p-typedopant selected from a group consisting of: Mg, Zn, Cd and Hg; a p-typedopant from Group IV of the periodic table; a p-type dopant selectedfrom a group consisting of: C and Si; and an n-type selected from agroup consisting of: Si, Ge, Sn, S, Se and Te; the doped semiconductoris doped during growth of the semiconductor.

In yet another embodiment, provided is a system for assembling one ormore elongated structures on a surface, wherein one or more of theelongated structures are at least one of the following: is at least oneof the following: a single crystal, an elongated and bulk-dopedsemiconductor that, at any point along its longitudinal axis, has alargest cross-sectional dimension less than 500 nanometers, and afree-standing and bulk-doped semiconductor with at least one portionhaving a smallest width of less than 500 nanometers, and wherein thesystem comprises: means for conditioning the surface with one or morefunctionalities that attract the one or more elongated structures toparticular positions on the surface, and means for aligning the one ormore elongated structures by attracting the one or more elongatedstructures to the particular positions using the one or morefunctionalities.

In additional aspects of this embodiment: at least one of the elongatedstructures are semiconductors; at least one of the elongated structuresare doped semiconductors; at least one of the elongated structures arebulk-doped semiconductors; at least one of the structures is a dopedsingle-crystal semiconductor; at least one of the structures is anelongated and bulk-doped semiconductor that, at any point along itslongitudinal axis, has a largest cross-sectional dimension less than 500nanometers; at least one of the structures is a free-standing andbulk-doped semiconductor with at least one portion having a smallestwidth of less than 500 nanometers; at least one of the structures is adoped semiconductor that is at least one of the following: a singlecrystal, an elongated and bulk- doped semiconductor that, at any pointalong its longitudinal axis, has a largest cross-sectional dimensionless than 500 nanometers, and a free-standing and bulk-dopedsemiconductor with at least one portion having a smallest width of lessthan 500 nanometers; the doped semiconductor comprises a semiconductorselected from a group consisting of: Si, Ge, Sn, Se, Te, B, Diamond, P,B—C, B—P(BP₆), B—Si, Si—C, Si—Ge, Si—Sn and Ge—Sn, SiC, BN/BP/BAs,AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb, BN/BP/BAs,AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,ZnO/ZnS/ZnSe/ZnTe, CdS/CdSe/CdTe, HgS/HgSe/HgTe, BeS/BeSe/BeTe/MgS/MgSe,GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr,CuI, AgF, AgCl, AgBr, AgI, BeSiN₂, CaCN₂, ZnGeP₂, CdSnAs₂, ZnSnSb₂,CuGeP₃, CuSi₂P₃, (Cu, Ag)(Al, Ga, In, Tl, Fe)(S, Se, Te)₂, Si₃N₄, Ge₃N₄,Al₂O₃, (Al, Ga, In)₂(S, Se, Te)₃, Al₂CO; the doped semiconductorcomprises a dopant selected from a group consisting of: a p-type dopantfrom Group III of the periodic table; an n-type dopant from Group V ofthe periodic table; a p-type dopant selected from a group consisting of:B, Al and In; an n-type dopant selected from a group consisting of: P,As and Sb; a p-type dopant from Group II of the periodic table; a p-typedopant selected from a group consisting of: Mg, Zn, Cd and Hg; a p-typedopant from Group IV of the periodic table; a p-type dopant selectedfrom a group consisting of: C and Si; and an n-type selected from agroup consisting of: Si, Ge, Sn, S, Se and Te; the doped semiconductoris doped during growth of the semiconductor.

In another embodiment, provided is a system for assembling a pluralityof elongated structures on a surface, wherein one or more of theelongated structures are at least one of the following: is at least oneof the following: a single crystal, an elongated and bulk-dopedsemiconductor that, at any point along its longitudinal axis, has alargest cross-sectional dimension less than 500 nanometers, and afree-standing and bulk-doped semiconductor with at least one portionhaving a smallest width of less than 500 nanometers, and wherein thesystem comprises means for depositing the plurality of elongatedstructures onto the surface; and means for electrically charging thesurface to produce electrostatic forces between two or more of theplurality of the elongated structures.

In additional aspects of this embodiment: at least one of the elongatedstructures are semiconductors; at least one of the elongated structuresare doped semiconductors; at least one of the elongated structures arebulk-doped semiconductors; at least one of the structures is a dopedsingle-crystal semiconductor; at least one of the structures is anelongated and bulk-doped semiconductor that, at any point along itslongitudinal axis, has a largest cross-sectional dimension less than 500nanometers; at least one of the structures is a free-standing andbulk-doped semiconductor with at least one portion having a smallestwidth of less than 500 nanometers; at least one of the structures is adoped semiconductor that is at least one of the following: a singlecrystal, an elongated and bulk-doped semiconductor that, at any pointalong its longitudinal axis, has a largest cross-sectional dimensionless than 500 nanometers, and a free-standing and bulk-dopedsemiconductor with at least one portion having a smallest width of lessthan 500 nanometers; the doped semiconductor comprises a semiconductorselected from a group consisting of: Si, Ge, Sn, Se, Te, B, Diamond, P,B—C, B—P(BP₆), B—Si, Si—C, Si—Ge, Si—Sn and Ge—Sn, SiC, BN/BP/BAs,AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb, BN/BP/BAs,AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,ZnO/ZnS/ZnSe/ZnTe, CdS/CdSe/CdTe, HgS/HgSe/HgTe, BeS/BeSe/BeTe/MgS/MgSe,GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr,CuI, AgF, AgCl, AgBr, AgI, BeSiN₂, CaCN₂, ZnGeP₂, CdSnAs₂, ZnSnSb₂,CuGeP₃, CuSi₂P₃, (Cu, Ag)(Al, Ga, In, Tl, Fe)(S, Se, Te)₂, Si₃N₄, Ge₃N₄,Al₂O₃, (Al, Ga, In)₂(S, Se, Te)₃, Al₂CO; the doped semiconductorcomprises a dopant selected from a group consisting of: a p-type dopantfrom Group III of the periodic table; an n-type dopant from Group V ofthe periodic table; a p-type dopant selected from a group consisting of:B, Al and In; an n-type dopant selected from a group consisting of: P,As and Sb; a p-type dopant from Group II of the periodic table; a p-typedopant selected from a group consisting of: Mg, Zn, Cd and Hg; a p-typedopant from Group IV of the periodic table; a p-type dopant selectedfrom a group consisting of: C and Si; and an n-type selected from agroup consisting of: Si, Ge, Sn, S, Se and Te; the doped semiconductoris doped during growth of the semiconductor.

In another embodiment, provided is a system for assembling a pluralityof elongated structures on a surface, wherein one or more of theelongated structures are at least one of the following: is at least oneof the following: a single crystal, an elongated and bulk-dopedsemiconductor that, at any point along its longitudinal axis, has alargest cross-sectional dimension less than 500 nanometers, and afree-standing and bulk-doped semiconductor with at least one portionhaving a smallest width of less than 500 nanometers, and wherein thesystem comprises: means for dispersing the one or more elongatedstructures on a surface of a liquid phase to form a Langmuir-Blodgettfilm; means for compressing the Langmuir-Blodgett film; and means fortransferring the compressed Langmuir-Blodgett film onto a surface.

In additional aspects of this embodiment: at least one of the elongatedstructures are semiconductors; at least one of the elongated structuresare doped semiconductors; at least one of the elongated structures arebulk-doped semiconductors; at least one of the structures is a dopedsingle-crystal semiconductor; at least one of the structures is anelongated and bulk-doped semiconductor that, at any point along itslongitudinal axis, has a largest cross-sectional dimension less than 500nanometers; at least one of the structures is a free-standing andbulk-doped semiconductor with at least one portion having a smallestwidth of less than 500 nanometers; at least one of the structures is adoped semiconductor that is at least one of the following: a singlecrystal, an elongated and bulk-doped semiconductor that, at any pointalong its longitudinal axis, has a largest cross-sectional dimensionless than 500 nanometers, and a free-standing and bulk-dopedsemiconductor with at least one portion having a smallest width of lessthan 500 nanometers; the doped semiconductor comprises a semiconductorselected from a group consisting of: Si, Ge, Sn, Se, Te, B, Diamond, P,B—C, B—P(BP₆), B—Si, Si—C, Si—Ge, Si—Sn and Ge—Sn, SiC, BN/BP/BAs,AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb, BN/BP/BAs,AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,ZnO/ZnS/ZnSe/ZnTe, CdS/CdSe/CdTe, HgS/HgSe/HgTe, BeS/BeSe/BeTe/MgS/MgSe,GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr,CuI, AgF, AgCl, AgBr, AgI, BeSiN₂, CaCN₂, ZnGeP₂, CdSnAs₂, ZnSnSb₂,CuGeP₃, CuSi₂P₃, (Cu, Ag)(Al, Ga, In, Tl, Fe)(S, Se, Te)₂, Si₃N₄, Ge₃N₄,Al₂O₃, (Al, Ga, In)₂(S, Se, Te)₃, Al₂CO; the doped semiconductorcomprises a dopant selected from a group consisting of: a p-type dopantfrom Group III of the periodic table; an n-type dopant from Group V ofthe periodic table; a p-type dopant selected from a group consisting of:B, Al and In; an n-type dopant selected from a group consisting of: P,As and Sb; a p-type dopant from Group II of the periodic table; a p-typedopant selected from a group consisting of: Mg, Zn, Cd and Hg; a p-typedopant from Group IV of the periodic table; a p-type dopant selectedfrom a group consisting of: C and Si; and an n-type selected from agroup consisting of: Si, Ge, Sn, S, Se and Te; the doped semiconductoris doped during growth of the semiconductor.

In another embodiment, provided is a system for assembling a pluralityof one or more elongated structures on a surface, wherein at least oneof the elongated structures are at least one of the following: a singlecrystal, an elongated and bulk-doped semiconductor that, at any pointalong its longitudinal axis, has a largest cross-sectional dimensionless than 500 nanometers, and a free-standing and bulk-dopedsemiconductor with at least one portion having a smallest width of lessthan 500 nanometers, and wherein the system comprises: means fordispersing the one or more elongated structures in a flexible matrix;means for stretching the flexible matrix in a direction to produce ashear force on the one or more elongated structures that causes the atleast one elongated structure to align in the direction; means forremoving the flexible matrix; and means for transferring the at leastone aligned elongated structure to a surface.

In additional aspects of this embodiment: at least one of the elongatedstructures are semiconductors; at least one of the elongated structuresare doped semiconductors; at least one of the elongated structures arebulk-doped semiconductors; at least one of the structures is a dopedsingle-crystal semiconductor; at least one of the structures is anelongated and bulk-doped semiconductor that, at any point along itslongitudinal axis, has a largest cross-sectional dimension less than 500nanometers; at least one of the structures is a free-standing andbulk-doped semiconductor with at least one portion having a smallestwidth of less than 500 nanometers; at least one of the structures is adoped semiconductor that is at least one of the following: a singlecrystal, an elongated and bulk-doped semiconductor that, at any pointalong its longitudinal axis, has a largest cross-sectional dimensionless than 500 nanometers, and a free-standing and bulk-dopedsemiconductor with at least one portion having a smallest width of lessthan 500 nanometers; the doped semiconductor comprises a semiconductorselected from a group consisting of: Si, Ge, Sn, Se, Te, B, Diamond, P,B—C, B—P(BP₆), B—Si, Si—C, Si—Ge, Si—Sn and Ge—Sn, SiC, BN/BP/BAs,AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb, BN/BP/BAs,AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,ZnO/ZnS/ZnSe/ZnTe, CdS/CdSe/CdTe, HgS/HgSe/HgTe, BeS/BeSe/BeTe/MgS/MgSe,GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr,CuI, AgF, AgCl, AgBr, AgI, BeSiN₂, CaCN₂, ZnGeP₂, CdSnAs₂, ZnSnSb₂,CuGeP₃, CuSi₂P₃, (Cu, Ag)(Al, Ga, In, Tl, Fe)(S, Se, Te)₂, Si₃N₄, Ge₃N₄,Al₂O₃, (Al, Ga, In)₂(S, Se, Te)₃, Al₂CO; the doped semiconductorcomprises a dopant selected from a group consisting of: a p-type dopantfrom Group III of the periodic table; an n-type dopant from Group V ofthe periodic table; a p-type dopant selected from a group consisting of:B, Al and In; an n-type dopant selected from a group consisting of: P,As and Sb; a p-type dopant from Group II of the periodic table; a p-typedopant selected from a group consisting of: Mg, Zn, Cd and Hg; a p-typedopant from Group IV of the periodic table; a p-type dopant selectedfrom a group consisting of: C and Si; and an n-type selected from agroup consisting of: Si, Ge, Sn, S, Se and Te; the doped semiconductoris doped during growth of the semiconductor.

The features and advantages of the embodiments described above and otherfeatures and advantages of these embodiments will be more readilyunderstood and appreciated from the detailed description below, whichshould be read together with the accompanying drawing Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, reference is madeto the accompanying drawings, which are incorporated herein by referenceand in which:

FIG. 1 is a perspective view of an example of a semiconductor article,or nanowire, in accordance with an embodiment of the invention;

FIG. 2 is a simplified schematic diagram of an example of a laserassisted catalytic growth process for fabrication of semiconductornanowires;

FIG. 3 is a schematic diagram that illustrates nanowire growth;

FIG. 4 is a schematic diagram that illustrates an example of a methodfor controlling nanowire diameter;

FIG. 5 is a schematic diagram that illustrates nanowire fabrication bydeposition on the edge of surface steps;

FIG. 6 is a schematic diagram that illustrates nanowire growth by vapordeposition in or on an elongated template;

FIGS. 7A-7E illustrate orthogonal assembly of semiconductor nanowires toform devices;

FIGS. 8A-8C show silicon nanowire current as a function of bias voltagefor different doping levels and gate voltages;

FIGS. 9A and 9B show silicon nanowire current as a function of biasvoltage for different phosphorous doping levels and gate voltages;

FIGS. 10A and 10B show energy band diagrams for p-type and n-typesilicon nanowire devices, respectively;

FIGS. 11A and 11B show temperature dependent current-voltage curvesrecorded on a heavily boron doped silicon nanowire;

FIG. 12 is a schematic diagram that depicts the use of monodispersedgold colloids as catalysts for the growth of well-defined GaPsemiconductor nanowires;

FIG. 13A shows a FE-SEM image of nanowires synthesized from 28.2nanometer colloids;

FIG. 13B shows a TEM image of another wire in the sample;

FIGS. 14A-14C show histograms of measured diameters for wires grown fromdifferent diameter colloids;

FIG. 14D shows a histogram of diameters for wires grown using theprevious method without colloids, in which the laser is used to bothgenerate the gold nanoclusters and the GaP reactants;

FIG. 15 shows a pseudobinary phase diagram for gold and galliumarsenide;

FIGS. 16A-16C show FE-SEM images of different nanowires prepared bylaser assisted catalytic growth;

FIG. 17A shows a diffraction contrast TEM image of an approximately 20nanometer diameter gallium arsenide nanowire;

FIGS. 17B-17D show high resolution TEM images of different diameternanowires;

FIG. 18A shows a FE-SEM image of CdSe nanowires prepared by laserassisted catalytic growth;

FIG. 18B shows a diffraction contrast TEM image of an 18 nanometerdiameter CdSe nanowire;

FIG. 18C shows a high resolution TEM image of an approximately 13nanometer diameter CdSe nanowire;

FIG. 19 is a schematic diagram showing GaN nanowire growth by laserassisted catalytic growth;

FIG. 20A shows a FE-SEM image of bulk GaN nanowire synthesized by laserassisted catalytic growth;

FIG. 20B shows a PXRD pattern recorded on bulk GaN nanowires;

FIG. 21A shows a diffraction contrast TEM image of a GaN nanowire thatterminates in a faceted nanoparticle of higher contrast;

FIG. 21B shows an HRTEM image of another GaN nanowire with a diameter ofapproximately 10 nanometers;

FIGS. 22A-22C illustrate doping and electrical transport of InPnanowires;

FIGS. 23A-23D illustrate crossed nanowire junctions and electricalproperties;

FIGS. 24A-24D illustrate optoelectrical characterization of nanowire P-Njunctions;

FIG. 25A shows an EL image taken from a p-type Si and n-type GaNnanojunction;

FIG. 25B shows current as a function of voltage for various gatevoltages;

FIG. 25C shows an EL spectrum for the nanojunction of FIGS. 25A;

FIGS. 26A-26D illustrate parallel and orthogonal assembly of nanowireswith electric fields;

FIGS. 27A-27F illustrate crossed silicon nanowire junctions;

FIGS. 28A-28D illustrate n⁺pn crossed silicon nanowire bipolartransistors;

FIGS. 29A-29D illustrate complementary inverters and tunnel diodes;

FIGS. 30A and 30B are schematics of fluidic channel structures for flowassembly;

FIGS. 31A-31D illustrate parallel assembly of nanowire arrays;

FIGS. 32A-32D illustrate assembly of periodic nanowire arrays; and

FIGS. 33A-33E illustrate layer-by-layer assembly and transportmeasurements of crossed nanowire arrays.

DETAILED DESCRIPTION

The present invention provides, in one aspect, techniques for controlleddoping of materials such as semiconductors at a very small spatialscale, and arrangement of doped materials in position relative to eachother to create useful devices. One set of embodiments involves dopingof a semiconductor, with a dopant (e.g., boron, aluminum, phosphorous,arsenic, etc.) selected according to whether an n-type or p-typesemiconductor is desired.

In various embodiments, this invention involves controlled doping ofsemiconductors selected from among indium phosphide, gallium arsenide,gallium nitride, cadmium selenide, and zinc selenide. Dopants including,but not limited to, zinc, cadmium, or magnesium can be used to formp-type semiconductors in this set of embodiments, and dopants including,but not limited to, tellurium, sulfur, selenium, or germanium can beused as dopants to form n-type semiconductors from these materials.These materials define direct band gap semiconductor materials and theseand doped silicon are well known to those of ordinary skill in the art.The present invention contemplates use of any doped silicon or directband gap semiconductor materials for a variety of uses.

As used herein, a “width” of an article is a distance of a straight linefrom a point on a perimeter of the article through the center of thearticle to another point on the perimeter of the article. As usedherein, a “width” or “cross-sectional dimension” at a point along thelongitudinal axis of an elongated article is a distance along a straightline that passes through the center of the cross-section at the pointand that connects two points on the perimeter of the cross section.

As used herein, an “elongated” article (e.g., semiconductor or sectionthereof) is an article for which, at any point along the longitudinalaxis of the article, a ratio of the length of the article to the largestwidth at the point is greater than 2:1.

As used herein, the “longitudinal axis” of an elongated article is anaxis along a largest dimension of the article.

As used herein, the “length” of an elongated article is a distance alongthe longitudinal axis from end to end of the article.

As used herein, a “longitudinal section” of an elongated article is aportion of the elongated article along the longitudinal axis of theelongated article than can have any length greater than zero and lessthan or equal to the length of the article.

As used herein, a “cross-section” at a point along the longitudinal axisof an elongated article is a plane at the point across the elongatedarticle that is orthogonal to the longitudinal axis of the article.

As used herein, a “cylindrical” article is an article having an exteriorshaped like a cylinder, but does not define or reflect any propertiesregarding the interior of the article.

In other words, a cylindrical article may have a solid interior or mayhave a hollowed-out interior.

As used herein, a “nanowire” or “NW” is an elongated semiconductor,i.e., a nanoscale semiconductor, that at any point along its length hasat least one cross-sectional dimension and, in some embodiments, twoorthogonal cross-sectional dimensions less than 500 nanometers,preferably less than 200 nanometers, more preferably less than 150nanometers, still more preferably less than 100 nanometers, even morepreferably less than 70, still more preferably less than 50 nanometers,even more preferably less than 20 nanometers, still more preferably lessthan 10 nanometers, and even less than 5 nanometers. The cross-sectionof the elongated semiconductor may have any arbitrary shape, including,but not limited to, circular, square, rectangular, elliptical. Regularand irregular shapes are included.

As used herein, a “nanotube” or “NT” is a nanowire that has ahollowed-out core.

As used herein, a “bulk-doped” article (e.g. semiconductor or sectionthereof) is an article for which a dopant is incorporated substantiallythroughout the crystalline lattice of the article, as opposed to anarticle in which a dopant is only incorporated in particular regions.For example, some articles such as carbon NTs typically are doped afterthe base material is grown, and thus the dopant only extends a finitedistance from the surface or exterior of the carbon NT into the interiorof the crystal line lattice. Further, carbon NTs are often combined asnested tubes forming alternating layers of base material and doped basematerial such that the dopant is not incorporated throughout the crystalline lattice of the base material.

As used herein to describe a “nanowire” or “NW”, “doped” meansbulk-doped. Accordingly, as used herein, a “doped nanowire” or “dopedNW” is a bulk-doped nanowire

As used herein, an “array” of articles (e.g., nanowires) comprises aplurality of the articles. As used herein, a “crossed array” is an arraywhere at least one of the articles contacts either another of thearticles or a signal node (e.g., an electrode).

As used herein, a first article (e.g., a nanowire or larger-sizedstructure) “coupled” to a second article is disposed such that the firstarticle either physically contacts the second article or is proximateenough to the second article to influence a property (e.g., electricalproperty, optical property, magnetic property) of the second article.

Thus, the present invention contemplates, in one aspect, an elongatedsemiconductor that has a smallest width of less than 500 nanometers thatis doped in any way (n-type or p-type). In other embodiments, thesemiconductor may have a smallest width less than about 200 nanometers,less than about 150 nanometers, or less than about 100 nanometers.Preferably, the semiconductor has a smallest width of less than about 80nanometers, more preferably less than about 70 nanometers, preferablyless than about 50 nanometers. Smaller widths, such as those with atleast one dimension of less than about 20 nanometers, less than about 10nanometers, or less than about 5 nanometers also are included. In someembodiments, two orthogonal cross-sectional dimensions of the elongatedsemiconductor may be less than the values given above. The aspect ratio,i.e., the ratio of semiconductor length to largest width, is greaterthan 2:1. In other embodiments, the aspect ratio may be greater than4:1, greater than 10:1, greater than 100:1 or even greater than 1000:1.Semiconductors such as these, at very small dimensions, find a varietyof uses as described below.

FIG. 1 is a perspective diagram illustrating an example of a cylindricalsemiconductor L1, for example, a wire-like semiconductor such as ananowire. The cylindrical semiconductor L1 has a length L2 and alongitudinal axis L3. At a point L5 along the longitudinal axis L3, thecylindrical semiconductor L1 has a plurality of widths L4 acrosscross-section L6, where one of the widths L4 is a smallest width at thepoint L5.

Such semiconductors may be free-standing. As used herein, a“free-standing” article is an article that at some point in its life isnot attached to another article or that is in solution.

Further, such a semiconductor may be a bulk-doped semiconductor. As usedherein, a “bulk-doped semiconductor” article (e.g. article or section ofan article) is a semiconductor for which a dopant is incorporatedsubstantially throughout the crystalline lattice of the semiconductor,as opposed to a semiconductor in which a dopant is only incorporated inparticular regions. For example, some semiconductors such as NTstypically are doped after the semiconductor is grown, and thus thedopant only extends a finite distance from the surface or exterior ofthe nanotube into the interior of the crystalline lattice. Further, NTsare often combined as nested tubes (i.e. cylinders) forming alternatinglayers of semiconductor and doped semiconductor such that the dopant isnot incorporated throughout the crystalline lattice of thesemiconductor. It should be understood that “bulk-doped” does not defineor reflect a concentration or amount of doping in a semiconductor, nordoes it indicate that the doping is necessarily uniform.

For a doped semiconductor, the semiconductor may be doped during growthof the semiconductor. Doping the semiconductor during growth may resultin the property that the doped semiconductor is bulk-doped. Further,such doped semiconductors may be controllably doped, such that aconcentration of a dopant within the doped semiconductor can becontrolled and therefore reproduced consistently, making possible thecommercial production of such semiconductors.

A variety of devices may be fabricated using semiconductors such asthose described above. Such devices include electrical devices, opticaldevices, mechanical devices or any combination thereof, includingopto-electronic devices and electromechanical devices.

In an embodiment a field effect transistor (FET) is produced using adoped semiconductor having a smallest width of less than 500 nanometersor other width described above. The doped semiconductor can be either ap-type or n-type semiconductor, as is known by those of ordinary skillin the art in FET fabrication. While FETs are known using nanotubes, tothe inventors' knowledge, prior arrangements select nanotubes at random,without control over whether the nanotube is metallic or semiconducting.In such a case a very low percentage of devices are functional, perhapsless than one in twenty, or one in fifty, or perhaps approximately onein one hundred. The present invention contemplates controlled doping ofnanowires such that a fabrication process can involve fabricatingfunctional FETs according to a technique in which much greater than onein fifty devices is functional. For example, the technique can involvedoping a nanowire, then fabricating an FET therefrom.

The invention also provides lightly-doped complementary inverters(complementary metal oxide semiconductors) arranged simply by contact ofan n-type semiconductor with a p-type semiconductor, for example byarrangement of crossed n-type and p-type semiconducting nanowires asshown below.

Also provided in accordance with the invention are tunnel diodes withheavily-doped semiconducting components. A tunnel diode can be arrangedsimilarly or exactly the same as a complementary inverter, with thesemiconductors being heavily doped rather than lightly doped. “Heavilydoped” and “lightly doped” are terms the meaning of which is clearlyunderstood by those of ordinary skill in the art.

One important aspect of the present invention is the ability tofabricate essentially any electronic device that can benefit fromadjacent n-type and p-type semiconducting components, where thecomponents are pre-fabricated (doped, in individual and separateprocesses with components separate from each other when doped) and thenbrought into contact after doping. This is in contrast to typical priorart arrangements in which a single semiconductor is n-doped in oneregion and p-doped in an adjacent region, but the n-type semiconductorregion and p-type semiconducting regions are initially adjacent prior todoping and do not move relative to each other prior to or after doping.That is, n-type and p-type semiconductors, initially in non-contactingarrangement, are brought into contact with each other to form a usefulelectronic device. Essentially any device can be made in accordance withthis aspect of the invention that one of ordinary skill in the art woulddesirably make using n-type and p-type semiconductors in combination.Examples of such devices include, but are not limited to, field effecttransistors (FETs), bipolar junction transistors (BJTs), tunnel diodes,complementary inverters, light emitting devices, light sensing devices,gates, inverters, AND, NAND, OR, and NOR gates, latches, flip-flops,registers, switches, clock circuitry, static or dynamic memory devicesand arrays, state machines, gate arrays, and any other dynamic orsequential logic or other digital devices including programmablecircuits. Also included are analog devices and circuitry, including butnot limited to, amplifiers, switches and other analog circuitry usingactive transistor devices, as well as mixed signal devices and signalprocessing circuitry.

Electronic devices incorporating semiconductor nanowires can becontrolled, for example, by electrical, optical or magnetic signals. Thecontrol may involve switching between two or more discrete states or mayinvolve continuous control of nanowire current, i.e., analog control. Inaddition to electrical signals, optical signals and magnetic signals,the devices may be controlled as follows:

-   -   (1) The device is switchable in response to biological and        chemical species, for example, DNA, protein, metal ions. In more        general sense, these species are charged or have dipole.    -   (2) The device is switchable in response to the mechanical        stretching, vibration and bending.    -   (3) The device is switchable in response to the temperature.    -   (4) The device is switchable in response to the environmental        pressure.    -   (5) The device is switchable in response to the movement of        environmental gas or liquid.

Many devices of the invention make particular use of crossed p/njunctions which can be junctions of crossed n-type and p-type nanowires.Crossed p/n junctions are defined by at least one n-type semiconductorand at least one p-type semiconductor, at least one portion of eachmaterial contacting at least one portion of the other material, and eachsemiconductor including portions that do not contact the othercomponent. They can be arranged by pre-doping the nanowires, thenbringing them into proximity with each other using techniques describedbelow.

Light-emission sources are provided in accordance with the invention aswell, in which electrons and holes combine, emitting light. One type oflight-emission source of the invention includes at least one crossed p/njunction, in particular, crossed p-type and n-type nanowires. In thisand other arrangements of the invention using crossed nanowires, thewires need not be perpendicular, but can be. When forward biased(positive charge applied to the p-type wire and a negative chargeapplied to the n-type wire) electrons flow toward the junction in then-type wire and holes flow toward the junction in the p-type wire. Atthe junction, holes and electrons combine, emitting light. Othertechniques may be used to cause one or more nanowires, or othersemiconductors to emit light, as described below in more detail.

At the size scale of the invention (nanoscale) the wavelength of lightemission can be controlled by controlling the size of at least one, andpreferably both components that are crossed to form the light-emittingjunction. For example, where nanowires are used, a nanowire with largersmallest dimension (broader wire) will provide emission at a lowerfrequency. For example, in the case of indium phosphide, at size scalesassociated with typical fabrication processes, the material emits at 920nanometers. At the size scales of the present invention the wavelengthof emission can be controlled to be at wavelengths shorter than 920nanometers, for example between 920 and 580 nanometers. Wavelengths canbe selected within this range, such as 900, 850, 800, 750, 700nanometers, etc., depending upon wire size.

Thus, one aspect of the invention involves a semiconductorlight-emission source that emits at a higher frequency than thesemiconductor causing emission emits in its bulk state such increase ofthe frequency of emission of light is often referred to herein asquantum confinement. “Bulk state”, in this context, means a state inwhich it is present as a component, or a portion of a component having asmallest dimension of greater than 500 nanometers. “Bulk state” also canbe defined as that state causing a material's inherent wavelength orfrequency of emission. The present invention provides for such controlover emission frequency of essentially any semiconducting or dopedsemiconducting material.

Assembly, or controlled placement of nanowires on surfaces can becarried out by aligning nanowires using an electrical field. Anelectrical field is generated between electrodes, nanowires arepositioned between the electrodes (optionally flowed into a regionbetween the electrodes in a suspending fluid), and will align in theelectrical field and thereby can be made to span the distance betweenand contact each of the electrodes.

In another arrangement individual contact points are arranged inopposing relation to each other, the individual contact points beingtapered to form a point directed towards each other. An electric fieldgenerated between such points will attract a single nanowire spanningthe distance between, and contacting each of, the electrodes. In thisway individual nanowires can readily be assembled between individualpairs of electrical contacts. Crossed-wire arrangements, includingmultiple crossings (multiple parallel wires in a first direction crossedby multiple parallel wires in a perpendicular or approximatelyperpendicular second direction) can readily be formed by firstpositioning contact points (electrodes) at locations where opposite endsof the crossed wires desirably will lie. Electrodes, or contact points,can be fabricated via typical microfabrication techniques.

These assembly techniques can be substituted by, or complemented with, apositioning arrangement involving positioning a fluid flow directingapparatus to direct fluid containing suspended nanowires toward and inthe direction of alignment with locations at which nanowires aredesirably positioned. A nanowire solution can be prepared as follows.After nanowires are synthesized, they are transferred into a solvent(e.g., ethanol), and then are sonicated for several seconds to severalminutes to obtain a stable suspension.

Another arrangement involves forming surfaces including regions thatselectively attract nanowires surrounded by regions that do notselectively attract them. For example, —NH₂ can be presented in aparticular pattern at a surface, and that pattern will attract nanowiresor nanotubes having surface functionality attractive to amines. Surfacescan be patterned using known techniques such as electron-beampatterning, “soft-lithography” such as that described in InternationalPatent Publication No. WO 96/29629, published Jul. 26, 1996, or U.S.Pat. No. 5,512,131, issued Apr. 30, 1996, each of which is incorporatedherein by reference. Additional techniques are described in U.S. PatentApplication Ser. No. 60/142,216, filed Jul. 2, 1999, by Lieber, et al.,incorporated herein by reference. Fluid flow channels can be created ata size scale advantageous for placement of nanowires on surfaces using avariety of techniques such as those described in International PatentPublication No. WO 97/33737, published Sep. 18, 1997, and incorporatedherein by reference. Other techniques include those described in U.S.patent application Ser. No. 09/578,589, filed May 25, 2000, andincorporated herein by reference.

FIGS. 7A-7E show one such technique for creating a fluid flow channelusing a polydimethyl siloxane (PDMS) mold. Channels can be created andapplied to a surface, and a mold can be removed and re-applied in adifferent orientation to provide a cross flow arrangement or differentarrangement.

The flow channel arrangement can include channels having a smallestwidth of less than 1 millimeter, preferably less than 0.5 millimeter,200 microns or less. Such channels are easily made by fabricating amaster by using photolithography and casting PDMS on the master, asdescribed in the above-referenced patent applications and internationalpublications. Larger-scale assembly is possible as well. The area thatcan be patterned with nanowire arrays is defined only by the feature ofthe channel which can be as large as desired.

Semiconductor nanowires have a crystalline core sheathed with 1-10 nmthick of amorphous oxide. This allows surface modification to terminatethe surface with various functional groups. For example, we can usemolecules, one end of which is alkyloxysilane group (e.g. —Si(OCH₃))reacting with nanowire surface, the other end of which comprise (1)—CH₃, —COOH, —NH₂, —SH, —OH, hydrazide, and aldehyde groups; (2) lightactivatable moieties: aryl azide, fluorinated aryl azide, benzophenoneetc. The substrate and electrodes are also modified with certainfunctional groups to allow nanowires to specifically bind or not bindonto the substrate/electrodes surface based on the their interaction.

Surface-functionalized nanowires can also be coupled to the substratesurface with functional cross-linkers, e.g. (1) Homobifunctionalcross-linkers, comprising homobifunctional NHS esters, homobifunctionalimidoesters, homobifunctional sulfhydryl-reactive linkers,difluorobenzene derivatives, homobifunctional photoactive linkers,homobifunctional aldehyde, bis-epoxides, homobifunctional hydrazide etc.(2) Heterobifunctional cross-linkers (3) Trifunctional cross-linkers.

The assembly of nanowires onto substrate and electrodes can also beassisted using bimolecular recognition. For example, we can immobilizeone biological binding partner onto the nanowire surface and the otherone onto substrate or electrodes using physical adsorption or covalentlylinking. Some good bio-recognitions are: DNA hybridization,antibody-antigen binding, biotin-avidin (or streptavidin) binding.

There are many techniques that may be used to grow bulk-dopedsemiconductors, such as nanowires, and for doping such nanowires duringgrowth.

For example, SiNWs (elongated nanoscale semiconductors) may besynthesized using laser assisted catalytic growth (LCG). As shown inFIGS. 2 and 3, laser vaporization of a composite target that is composedof a desired material (e.g. InP) and a catalytic material (e.g. Au)creates a hot, dense vapor which quickly condenses into liquidnanoclusters through collision with the buffer gas. Growth begins whenthe liquid nanoclusters become supersaturated with the desired phase andcontinues as long as the reactant is available. Growth terminates whenthe nanowires pass out of the hot reaction zone or when the temperatureis turned down. Au is generally used as catalyst for growing a widerange of elongated nanoscale semiconductors. However, The catalyst isnot limited to Au only. A wide rage of materials such as (Ag, Cu, Zn,Cd, Fe, Ni, Co . . . ) can be used as the catalyst. Generally, any metalthat can form an alloy with the desired semiconductor material, butdoesn't form more stable compound than with the elements of the desiredsemiconductor can be used as the catalyst. The buffer gas can be Ar, N₂,and others inert gases. Sometimes, a mixture of H₂ and buffer gas isused to avoid un-desired oxidation by residue oxygen. Reactive gas canalso be introduced when desired (e.g. ammonia for GaN). The key point ofthis process is laser ablation generates liquid nanoclusters thatsubsequently define the size and direct the growth direction of thecrystalline nanowires. The diameters of the resulting nanowires aredetermined by the size of the catalyst cluster, which in turn can bevaried by controlling the growth conditions (e.g. background pressure,temperature, flow rate . . . ). For example, lower pressure generallyproduces nanowires with smaller diameters. Further diameter control canbe done by using uniform diameter catalytic clusters.

With same basic principle as LCG, if uniform diameter nanoclusters (lessthan 10-20% variation depending on how uniform the nanoclusters are) areused as the catalytic cluster, nanowires with uniform size (diameter)distribution can be produced, where the diameter of the nanowires isdetermined by the size of the catalytic clusters, as illustrated in FIG.4. By controlling the growth time, nanowires with different lengths canbe grown.

With LCG, nanowires can be flexibly doped by introducing one or moredopants into the composite target (e.g. (Ge for n-type doping of InP).The doping concentration can be controlled by controlling the relativeamount of doping element, typically 0-20%, introduced in the compositetarget.

Laser ablation may be used as the way to generate the catalytic clustersand vapor phase reactant for growth of nanowires and other relatedelongated nanoscale structures. but fabrication is not limited to laserablation. Many ways can be used to generate vapor phase and catalyticclusters for nanowire growth (e.g. thermal evaporation). Anothertechnique that may be used to grow nanowires is catalytic chemical vapordeposition (C-CVD). C-CVD utilizes the same basic principles as LCG,except that in the C-CVD method, the reactant molecules (e.g., silaneand the dopant) are from vapor phase molecules (as opposed to vaporsource from laser vaporization.

In C-CVD, nanowires can be doped by introducing the doping element intothe vapor phase reactant (e.g. diborane and phosphane for p-type andn-type doped nanowire). The doping concentration can be controlled bycontrolling the relative amount of the doping element introduced in thecomposite target. It is not necessary to obtain elongated nanoscalesemiconductors with the same doping ratio as that in the gas reactant.However, by controlling the growth conditions (e.g. temperature,pressure . . . ), nanowires with same doping concentration can bereproduced. And the doping concentration can be varied over a largerange by simply varying the ratio of gas reactant (e.g. 1 ppm-10%).

There are several other techniques that may be used to grow elongatednanoscale semiconductors such as nanowires. For example, nanowires ofany of a variety of materials can be grown directly from vapor phasethrough a vapor-solid process. Also, nanowires can also be produced bydeposition on the edge of surface steps, or other types of patternedsurfaces, as shown in FIG. 5. Further, nanowires can be grown by vapordeposition in/on any general elongated template, for example, as shownin FIG. 6. The porous membrane can be porous silicon, anodic alumina ordiblock copolymer and any other similar structure. The natural fiber canbe DNA molecules, protein molecules carbon nanotubes, any otherelongated structures. For all the above described techniques, the sourcematerials can be came from a solution phase rather than a vapor phase.While in solution phase, the template can also be column micelles formedby surfactant molecules in addition to the templates described above.

Using one or more of the above techniques, elongated nanoscalesemiconductors, including semiconductor nanowires and dopedsemiconductor nanowires, can be grown. Such bulk-doped semiconductorsmay include various combinations of materials, including semiconductorsand dopants. The following are non-comprehensive lists of suchmaterials. Other materials may be used. Such materials include, but arenot limited to:

Elemental semiconductors:

Si, Ge, Sn, Se, Te, B, Diamond, P

Solid solution of Elemental semiconductors:

B—C, B—P(BP₆), B—Si, Si—C, Si—Ge, Si—Sn, Ge—Sn

IV-IV group semiconductors:

SiC

III-V semiconductors:

BN/BP/BAs, AlN/AlP/AlAs/AlSb, GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb,

Alloys of III-V Group:

any combination of two or more of the above compound (e.g.: AlGaN,GaPAs, InPAs, GaInN, AlGaInN, GaInAsP . . . )

II-VI semiconductors:

ZnO/ZnS/ZnSe/ZnTe, CdS/CdSe/CdTe, HgS/HgSe/HgTe, BeS/BeSe/BeTe/MgS/MgSe

Alloys of II-VI Group: any combination of two or more of the abovecompound (e.g.: (ZnCd)Se, Zn(SSe) . . . )Alloy of II-VI and III-V semiconductors:

combination of any one II-VI and one III-V compounds,eg.(GaAs)_(x)(ZnS)_(1-x)

IV-VI semiconductors:

GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe

I-VII semiconductors:

CuF, CuCl, CuBr, CuI, AgF, AgCl, AgBr, AgI

Other semiconductor compounds

II-IV-V₂: BeSiN₂, CaCN₂, ZnGeP₂, CdSnAs₂, ZnSnSb₂ . . .

I-IV₂-V₃: CuGeP₃, CuSi₂P₃ . . .

I-III-VI₂: (Cu, Ag)(Al, Ga, In, Tl, Fe)(S, Se, Te)₂

IV₃-V₄: Si₃N₄, Ge₃N₄ . . .

III₂-VI₃: Al₂O₃, (Al, Ga, In)₂(S, Se, Te)₃ . . .

III₂-IV-VI: Al₂CO . . .

For Group IV semiconductor materials, a p-type dopant may be selectedfrom Group III, and an n-type dopant may be selected from Group V. Forsilicon semiconductor materials, a p-type dopant may be selected fromthe group consisting of B,

Al and In, and an n-type dopant may be selected from the groupconsisting of P, As and Sb. For Group III-V semiconductor materials, ap-type dopant may be selected from Group II, including Mg, Zn, Cd andHg, or Group IV, including C and Si. An n-type dopant may be selectedfrom the group consisting of Si, Ge, Sn, S, Se and Te. It will beunderstood that the invention is not limited to these dopants.

Examples Doping and Electrical Transport in Nanowires

Single crystal n-type and p-type silicon nanowires (SiNWs) have beenprepared and characterized by electrical transport measurements. As usedherein, a “single crystal” item is an item that has covalent bonding,ionic bonding, or a combination thereof throughout the item. Such asingle crystal item may include defects in the crystal, but isdistinguished from an item that includes one or more crystals, notionically or covalently bonded, but merely in close proximity to oneanother. Laser catalytic growth was used to introduce controllablyeither boron or phosphorous dopants during the vapor phase growth ofSiNWs. Two terminal, gate-dependent measurements made on individualboron-doped and phosphorous-doped SiNWs show that these materials behaveas p-type and n-type materials, respectively. Estimates of the carriermobility made from gate-dependent transport measurements are consistentwith diffusive transport. In addition, these studies show it is possibleto heavily dope SiNWs and approach a metallic regime.Temperature-dependent measurements made on heavily doped SiNWs show noevidence for coulomb blockade at temperature down to 4.2 K, and thustestify to the structural and electronic uniformity of the SiNWs.Potential applications of the doped SiNWs are discussed. Currently,there is intense interest in one dimensional (1D) nanostructures, suchas nanowires and nanotubes, due to their potential to test fundamentalconcepts about how dimensionality and size affect physical properties,and to serve as critical building blocks for emerging nanotechnologies.Of particular importance to 1D nanostructures is the electricaltransport through these “wires”, since predictable and controllableconductance will be critical to many nanoscale electronics applications.To date, most efforts have focused on electrical transport in carbonnanotubes. These studies have shown interesting fundamental features,including the existence of coherent states extending over hundreds ofnanometers, ballistic conduction at room temperature, and Luttingerliquid behavior, and have demonstrated the potential for devices such asfield effect transistors. However, there are important limitations ofnanotubes. First, the specific growth of metallic or semiconductingtubes, which depends sensitively on diameter and helicity, is notpossible. Studies dependent on the specific conducting behavior mustthus rely on chance observation. Second, controlled doping ofsemiconducting nanotubes is not possible, although it is potentiallycritical for devices applications. Semiconductor nanowires, however, canovercome these limitations of carbon nanotubes. These nanowires willremain semiconducting independent of diameter, and moreover, it shouldbe possible to take advantage of the vast knowledge from thesemiconductor industry to dope the nanowires.

To this end, we here report the first demonstration of controlled dopingof SiNWs and the characterization of the electrical properties of thesedoped nanowires using transport measurements. Gate-dependent, twoterminal measurements demonstrate that boron-doped (B-doped) andphosphorous-doped (P-doped) SiNWs behave as p-type and n-type materials,respectively, and estimates of the carrier mobilities suggest diffusivetransport in these nanowires. In addition, temperature dependentmeasurements made on heavily doped SiNWs show no evidence for coulombblockade at temperatures down to 4.2 K.

SiNWs were synthesized using the laser-assisted catalytic growth (LCG)we have described previously. Briefly, a Nd-YAG laser (532 nm; 8 nspulse width, 300 mJ/pulse, 10 Hz) may be used to ablate a gold target,which produces gold nanocluster catalyst particles within a reactor. TheSiNWs may be grown in a flow of SiH₄ as the reactant. Such SiNWs may bedoped with boron by incorporating B₂H₆ in the reactant flow, and may bedoped with phosphorous using a Au-P target (99.5:0.5 wt %, Alfa Aesar)and additional red phosphorous (99%, Alfa Aesar) at the reactant gasinlet. Transmission electron microscopy (TEM) measurements demonstratethat doped SiNWs grown using this technique have a single crystalsilicon core that is covered by a dense SiO_(x) sheath as describedpreviously.

Electrical contact to individual SiNWs were made using standard electronbeam lithography methods using a JEOL 6400 writer. The nanowires weresupported on oxidized Si substrate (1-10 Ω-cm resistivity, 600 nm SiO₂,Silicon Sense, Inc.) with the underlying conducting Si used as a backgate. The contacts to the SiNWs were made using thermally evaporated Al(50 nm) and Au (150 nm). Electrical transport measurements were madeusing a homebuilt system with less than or equal to 1 pA noise undercomputer control. The temperature-dependent measurements were made in aQuantum Design magnetic property measurement system.

TEM studies show that the boron and phosphorous-doped SiNWs are singlecrystals, although these measurements do not have sufficient sensitivityto quantify the boron or phosphorous doping level in individual wires.We can, however, demonstrate unambiguously the presence of p-type(boron) or n-type (phosphorous) dopants and the relative doping levelsusing electrical transport spectroscopy. In these measurements, a gateelectrode is used to vary the electrostatic potential of the SiNW whilemeasuring current versus voltage of the nanowire. The change inconductance of SiNWs as function of gate voltage can be used todistinguish whether a given nanowire is p-type or n-type since theconductance will vary oppositely for increasing positive (negative) gatevoltages.

Typical gate-dependent current versus bias voltage (I-V) curves recordedon intrinsic and B-doped SiNWs are shown in FIGS. 8A-8C. The two B-dopedwires shown in FIGS. 8B and 8C were synthesized using SiH₄:B₂H₆ ratiosof 1000:1 and 2:1, respectively. In general, the two terminal I-V curvesare linear and thus suggest that the metal electrodes make ohmiccontacts to the SiNWs. The small nonlinearity observed in the intrinsicnanowire indicates that this contact is slightly nonohmic. Analysis ofI-V data, recorded at zero gate voltage (V_(g)=0), which accounts forcontributions from the contact resistance and oxide coating on the SiNW,yield a resistivity of 3.9×10² Ω-cm. Significantly, when V_(g) is madeincreasingly negative (positive), the conductance increases (decreases).This gate dependence shows that the SiNW is a p-doped semiconductor(discussion below). Similar I-V versus V_(g) curves were recorded forthe lightly B-doped SiNW, and show that it is also p-type. Moreover, theV_(g)=0 resistivity of this B-doped SiNW (1 Ω-cm) is more than twoorders of magnitude smaller than the intrinsic SiNW, and demonstratesclearly our ability to control conductivity chemically. This latterpoint is further supported by I-V measurements on the heavily B-dopedSiNWs show in FIG. 8C. This wire has a very low resistivity of 6.9×10⁻³Ω-cm and shows no dependence on V_(g); that is, I-V data recorded withV_(g) of 0 and 20 V are overlapping. These results are consistent with ahigh carrier concentration that is near the metallic limit.

We have also measured V_(g)-dependent transport in lightly and heavilyP-doped SiNWs. The I-V recorded on the lightly doped nanowire (FIG. 9A)is somewhat nonlinear, which indicates nonideal contact between theelectrodes and nanowire, and the V_(g) dependence is opposite of thatobserved for the B-doped SiNWs. Significantly, this observed gatedependence is consistent with n-type material as expected for P-doping.The estimated resistivity of this wire at V_(g)=0 is 2.6×10² Ω-cm. Thisrelatively high resistivity is suggestive of a low doping level and/orlow mobility. In addition, heavily P-doped SiNWs have also been made andstudied. The I-V data recorded on a typical heavily P-doped wire arelinear, have a resistivity of 2.3×10⁻² Ω-cm, and shows no dependence onV_(g). The low resistivity (four orders of magnitude smaller than thelightly P-doped sample) and V_(g) independence demonstrate that highcarrier concentrations can also be created via P-doping of the SiNWs.

The above results demonstrate that boron and phosphorous can be used tochange the conductivity of SiNWs over many orders of magnitude and thatthe conductivity of the doped SiNWs respond oppositely to positive(negative) V_(g) for boron and phosphorous dopants. Indeed, theV_(g)-dependence provides strong proof for p-type (holes) doping withboron and n-type (electrons) doping with phosphorous in the SiNWs. Theobserved gate dependencies can be understood by referring to theschematics shown in FIGS. 10A and 10B, which show the effect of theelectrostatic potential on the SiNW bands. In these diagrams, a p-typenanowire (a) and n-type nanowire (b) are contacted at both ends to metalelectrodes. As for a conventional metal-semiconductor interface, theSiNW bands bend (up for p-type; down for n-type) to bring the nanowireFermi level in line with that of the metal contacts. When V_(g)>0, thebands are lowered, which depletes the holes in B-doped SiNWs andsuppress conductivity, but leads to an accumulation of electrons inP-doped SiNWs and enhance the conductivity. Conversely, V_(g)<0 willraise the bands and increase the conductivity of B-doped (p-type) SiNWsand decrease the conductivity of the P-doped (n-type) nanowires.

In addition, it is possible to estimate the mobility of carriers fromthe transconductance, dI/dV_(g)=μC/L²) V, where μ is the carriermobility, C is the capacitance, and L is the length of the SiNW. TheSiNW capacitance is given by C≈2πεε_(O)L/ln(2h/r), where c is thedielectric constant, h is the thickness of the silicon oxide layer, andr is the SiNW radius. Plots of dI/dV_(g) versus V were found to belinear for the intrinsic (FIG. 8A) and lightly B-doped (FIG. 8B) SiNWs,as expected for this model. The slopes of dI/dV_(g) for the intrinsic(2.13×10⁻¹¹) and B-doped (9.54×10⁻⁹) SiNW yield mobilities of 5.9×10⁻³cm²/V-s and 3.17 cm²/V-s, respectively. The mobility for the B-dopednanowire is comparable to that expected in bulk Si at a dopingconcentration of 10²⁰ cm⁻³. We also note that the mobility is expectedto increase with decreasing dopant concentration, although in ourintrinsic (low dopant concentration) SiNW the mobility is extremely low.It is possible that the reduced mobility is due to enhanced scatteringin the smaller diameter (intrinsic) SiNW. We believe that future studiesof the mobility as a function of diameter (for constant dopantconcentration) should illuminate this important point.

Lastly, we have carried out preliminary temperature-dependent studies ofheavily B-doped SiNWs. Temperature dependent I-V curves show that theconductance decreases with decreasing temperature, as expected for adoped semiconductor (FIGS. 11A and 11B). More importantly, we see noevidence for a coulomb blockade down to our lowest accessibletemperature (FIG. 11B). The small nonlinearity near V=0 is attributed toa contact effect since high resolution I-V versus V_(g) measurementsshow no signature for coulomb blockade. Coulomb charging effect in thishomogenous wire between the electrodes (a 150 nm thick 2.3 μm long wire)would require a temperature below about 26 mK estimated from kT=e²/2C.This indicates strongly that variations in SiNW diameter and defects aresufficiently small that they do not effectively “break up” the SiNW intosmall islands, which would exhibit coulomb blockade at thesetemperatures. These results contrast studies of lithographicallypattered SiNWs, which show coulomb blockade, and testify to the highquality of our free standing nanowires.

Single crystal n-type and p-type silicon nanowires (SiNWs) have beenprepared and characterized by electrical transport measurements. Lasercatalytic growth was used to introduce controllably either boron orphosphorous dopants during the vapor phase growth of SiNWs.Two-terminal, gate-dependent measurements made on individual boron-dopedand phosphorous-doped SiNWs show that these materials behave as p-typeand n-type materials, respectively. Estimates of the carrier mobilitymade from gate-dependent transport measurements are consistent withdiffusive transport, and show an indication for reduced mobility insmaller diameter wires. In addition, these studies show it is possibleto incorporate high dopant concentrations in the SiNWs and approach themetallic regime. Temperature-dependent measurements made on heavilydoped SiNWs show no evidence for single electron charging attemperatures down to 4.2 K, and thus suggest that the SiNWs possess ahigh degree of structural and doping uniformity.

We believe that our successful doping of SiNWs to create n-type andp-type materials will open up exciting opportunities in nanoscalescience and technology. Doped SiNWs will be candidates for investigatingfundamental issues of transport in 1D nanostructures. The structuresstudies in this paper are also field effect transistors (FETs), and itwill be possible using self-assembly techniques to integrate many SiNWFETs into structures perhaps for nanoelectronics applications. It shouldalso be possible to combine p-type and n-type SiNWs, for example incrossed arrays, to create p-n junctions that could also be consideredfor devices and sensors in the future.

Crossed SiNW p-n junctions have been formed by directed assembly ofp-type (n-type) SiNWs over n-type (p-type) SiNWs. Transport measurementsexhibit rectification in reverse bias and a sharp current onset inforward bias. Simultaneous measurements made on the p-type and n-typeSiNWs making up the junction demonstrate that the contacts to thesenanowires are ohmic (nonrectifying), and thus that the rectifyingbehavior is due to the p-n junction between the two SiNWs.

FIG. 8A shows current (I) vs bias voltage (V) curves recorded on a 70 nmdiameter intrinsic SiNW at different gate voltages (V_(g)). Curves 1, 2,3, 4, 5, 6, and 7 correspond to

V_(g)=−30, −20, −10, 0V, 10, 20, and 30 V, respectively. The inset is atypical scanning electron micrograph of the SiNW with metal contacts(scale bar=10 μm). FIG. 8B shows I-V data recorded on a 150 nm diameterB-doped SiNW; curves 1-8 correspond to V_(g)=−20, −10, −5, 0, 5, 10, 15and 20 V, respectively. FIG. 8C shows I-V curves recorded on a 150 nmdiameter heavily B-doped SiNW; V_(g)=20 V (solid line) and 0 V (heavydashed line).

FIG. 9A shows I-V data recorded on a 60 nm diameter P-doped SiNW. Curves1, 2, 3, 4, 5, and 6 correspond to V_(g)=20, 5, 1, 0, −20, and −30 V,respectively. FIG. 9B shows I-V curves recorded on a 90 nm diameterheavily P-doped SiNW; V_(g)=0 V (solid line) and −20 V (heavy dashline).

FIG. 10A shows energy band diagrams for p-type SiNW devices. FIG. 10Bshows energy band diagrams for n-type SiNW devices. The diagrams showschematically the effect of V_(g) on the electrostatic potential forboth types of nanowires.

FIGS. 11A and 11B show temperature dependent I-V curves recorded on aheavily B-doped SiNW. In FIG. 11A, curves 1, 2, 3, 4, 5, and 6correspond to temperatures of 295, 250, 200, 150, 100, and 50 K,respectively. FIG. 11B shows I-V data recorded on the nanowire at 4.2 K.

Diameter Selective Synthesis of Semiconductor Nanowires

Nearly monodisperse samples of single crystalline GaP nanowires havebeen synthesized with diameters of 10, 20, and 30 nm and lengths greaterthan 10 μm by exploiting well-defined gold colloids as catalysts in ourlaser catalytic growth (LCG) process. In this method, the Ga and Preactants generated by laser ablation of solid GaP are subsequentlydirected into a nanowire structure by gold nanocluster catalysts.Transmission electron microscopy (TEM) studies of nanowires prepared inthis way demonstrate that the distributions of nanowire diameters aredefined by those of the nanocluster catalysts. High-resolution TEM showsthat the wires are a single crystal zinc blend with a [111] growthdirection, and energy dispersive X-ray analysis confirms that thenanowire composition is stoichiometric GaP. The use of monodispersenanocluster catalysts combined with the LCG method will enable thegrowth of a wide range of semiconductor nanowires with well-defined andcontrolled diameters, and thus opens up opportunities from fundamentalproperties of one-dimensional (1D) systems to the assembly of functionalnanodevices.

Nearly monodisperse samples of single crystalline GaP nanowires havebeen synthesized with diameters of 10, 20, and 30 nm and lengths greaterthan 10 μm by exploiting well-defined gold colloids as catalysts in ourlaser catalytic growth (LCG) synthetic methodology. Transmissionelectron microscopy (TEM) studies of nanowires prepared in this waydemonstrate that the distributions of nanowire diameters are defined bythose of the nanocluster catalysts. High-resolution TEM shows that thewires are a single crystal zinc blende with a [111] growth direction,and energy dispersive X-ray analysis (EDAX) confirms that the nanowirecomposition is stoichiometric GaP.

Recent interest in low-dimensional semiconductor materials has beenmotivated by the push for miniaturization of electronic andoptoelectronic devices and a need to understand the fundamentals ofnanoscale chemistry and physics. In particular, one-dimensional (1D)systems are exciting from both fundamental and applied viewpoints.Fascinating physical phenomena, such as Luttinger liquid behavior, andnumerous applications from interconnects to scanning probe microscopies,require high-quality, well-defined 1D nanostructures. Experimentalprogress in the field of 1D nanostructures has often been limited by theability to create new materials in this size regime with controlledsize, structure, and composition.

Early approaches to 1D nanostructure synthesis employed thin film growthand lithographic techniques. In particular, “T-wires” have beenfabricated by growing semiconductor quantum wells via molecular beamepitaxy, followed by cleavage and overgrowth on the cleaved surface,while “V-groove” nanowires have been prepared by etching trenches on asurface and then depositing a small amount of material into theresulting grooves. One of the significant limitations of theseapproaches is that the nanowires are embedded in a substrate, whichprecludes the assembly of complex 2D and 3D nanostructures. Templateapproaches have also been used for growing a wide-range of nanowires.These methods can provide good control over the length and diameter ofnanowires, although they are limited in that polycrystalline structuresare often produced.

Our laboratory has made significant progress toward the development of ageneral synthetic approach to free-standing single-crystal semiconductornanowires via the LCG method. In LCG, laser ablation of a solid targetis used to simultaneously generate nanoscale metal catalyst clusters andreactive semiconductor atoms that produce nanowires via avapor-liquid-solid growth mechanism. This method has been used toproduce a wide range of group IV, III-V, and II-VI nanowires. We havesuggested that the size of the catalyst nanocluster determines the sizeof the wire during growth, and thus one can envision creating wires witha narrow size distribution by exploiting monodisperse catalystnanoclusters (FIG. 12). Here we utilize nanometer diameter gold colloidsto explore this approach.

GaP nanowires were grown by LCG using 8.4, 18.5, and 28.2 nm diametergold colloids. In these experiments the catalyst nanoclusters aresupported on a SiO₂ substrate and laser ablation is used to generate theGa and P reactants from a solid target of GaP. Field emission scanningelectron microscopy (FESEM) demonstrates that nanowires with lengthsexceeding 10 μm (FIG. 13A) were obtained using all three sizes ofcatalyst. Examination of the nanowire ends also shows the presence ofthe nanocluster catalyst (FIG. 13A, inset). Control experiments carriedout without the Au colloids did not produce nanowires. The FESEM imagesshow that the nanowire diameter distributions are narrower than obtainedin experiments without the colloid catalysts, although FESEM is not agood method for quantifying these distributions since small variationsin the focal plane can produce significant changes in the observeddiameter.

The growth apparatus used in these experiments is similar to thatreported. Substrates were made by placing a silicon wafer with 600 nm ofthermal oxide (Silicon Sense) into a solution of 95:5 EtOH:H₂O with 0.4%N-[3-(Trimethoxysilyl)propyl]-ethylenediamine for 5 minutes, followed bycuring at 100-110° C. for 10 minutes. Solutions of Au colloids (TedPella) were diluted to concentrations of 10⁹-10¹¹ particles/mL tominimize aggregation and were deposited on the substrates. Substrateswere placed in a quartz tube at the downstream end of the furnace with asolid target of GaP placed 3-4 cm outside of the furnace at the upstreamend. The chamber was evacuated to less than 100 mTorr, and thenmaintained at 250 Torr with an Airflow of 100 sccm. The furnace washeated to 700° C. and the target was ablated for 10 minutes with an ArFexcimer laser (λ=193 nm, 100 mJ/pulse, 10 Hz). After cooling, thesubstrates were examined by FESEM (LEO 982). For TEM (JEOL 200CX and2010) and EDAX analysis, nanowires were deposited onto copper gridsafter removal from the substrates by sonication in ethanol.

To obtain a quantitative measure of the nanowire diameter distributionsproduced using the gold colloids, and to better characterize theirstructure and composition, we used TEM. High resolution TEM shows thatthe wires are single crystal (FIG. 13B), growing in the [111] direction,and EDAX confirms the composition to be stoichiometric GaP (Ga:P1.00:0.94), within the limits of this technique. Significantly,extensive TEM analysis of nanowire diameters demonstrates the extremelygood correlation with the colloid catalyst diameters and dispersion(FIGS. 14A and 14B); that is, for wires grown from 28.2±2.6, 18.5±0.9,and 8.4±0.9 nm colloids we observe mean diameters of 30.2±2.3, 20.0±1.0,and 11.4±1.9 nm, respectively. The mean nanowire diameter is generally1-2 nm larger than that of the colloids. We believe that this increaseis due to alloying of the Ga and P reactants with the colloids beforenucleation of the nanowire occurs. For the 30 nm and 20 nm wires (FIGS.14A and 14B) it is clear that the width of the nanowire distributionsmirrors those of the colloid, suggesting that the monodispersity of thewires is limited only by the dispersity of the colloids. For the 10 nmdiameter wires (FIG. 14C), a small broadening (1 nm) of the wiredistribution can be attributed to aggregation of the colloids.

The mean diameter and distribution width increased as more concentratedsolutions of the colloid were dispersed onto the substrate. The factthat the distribution has peaks separated by ˜2.5 nm suggests that someof the wires grow from aggregates of two colloids, although additionalwork is required to substantiate this point. In all cases, thedistribution of wire diameters is more than an order of magnitudenarrower than those grown without the use of colloid catalyst (FIG.14D): 43±24 nm.

We believe that this work demonstrates clearly for the first time anability to exert systematic control over the diameter of semiconductornanowires for a variety of colloids. Previous attempts to grow nanowireson surfaces with poorly defined catalysts resulted in nanowires withnon-uniform diameters greater than 50 nm. Other attempts to control thediameter of nanowires by varying the background carrier gas merelyshifted the mean diameter of the wires slightly and yielded much broaderdistributions of wires than we have achieved with colloid-mediatedgrowth.

In summary, we have demonstrated the controlled synthesis ofsemiconductor wires with monodisperse diameter distributions. Thesehigh-quality, single crystalline wires represent good candidates forboth further studies of low-dimensional physics as well as forapplications in various fields of nanoscale science and technology. Inparticular, we believe that the synthesis of controlled diameter sampleswill greatly facilitate the assembly of these nanoscale building blocksinto complex and functional 2D and 3D nanosystems.

FIG. 12 is a schematic depicting the use of monodisperse gold colloidsas catalysts for the growth of well-defined GaP semiconductor nanowires.

FIG. 13A shows a FESEM image of nanowires synthesized from 28.2 nmcolloids (scale bar is 5 μm). The inset is a TEM image of the end of oneof these wires (scale bar is 50 nm). The high contrast featurecorresponds to the colloid catalyst at the end of the wire. FIG. 13Bshows a TEM image of another wire in this sample (scale bar is 10 nm).The [111] lattice planes are resolved, showing that wire growth occursalong this axis, in agreement with earlier work. Measurement of theinter-plane spacing gives a lattice constant of 0.54 nm (±0.05 nm) forthe wire, in agreement with the bulk value for GaP, 0.5451 nm.

FIGS. 14A-14C show histograms of measured diameters for wires grown from28.2 nm (FIG. 14A), 18.5 nm (FIG. 14B), and 8.4 nm (FIG. 14C) colloids.The solid line shows the distribution of wire. FIG. 14D shows ahistogram of diameters for wires grown using the previous method withoutcolloids, in which the laser is used to both generate the Aunanoclusters and the GaP reactants. The distribution is very broad(stan. dev. 23.9 nm) and the mean diameter (42.7 nm) greater than thosesynthesized using the predefined colloid catalyst. In all cases, thereported nanowire diameters correspond to the crystalline cores. Theamorphous oxide layers on the surface of all nanowires are relativelyuniform from wire to wire within the same experiment, but vary from 2-6nm in thickness between syntheses.

General Synthesis of Compound Semiconductor Nanowires

The predictable synthesis of a broad range of multicomponentsemiconductor nanowires has been accomplished using laser-assistedcatalytic growth. Nanowires of binary group III-V materials (GaAs, GaP,InAs and InP), ternary III-V materials (GaAs/P, InAs/P), binary II-VIcompounds (ZnS, ZnSe, CdS, and CdSe) and binary SiGe alloys have beenprepared in bulk quantities as high purity (>90%) single crystals. Thenanowires have diameters varying from three to tens of nanometers, andlengths extending to tens of micrometers. The synthesis of this widerange of technologically important semiconductor nanowires can beextended to many other materials and opens up significant opportunitiesin nanoscale science and technology.

The synthesis of nanoscale materials is critical to work directedtowards understanding fundamental properties of small structures,creating nanostructured materials and developing nanotechnologies.Nanowires and nanotubes have been the focus of considerable attention,because they have the potential to answer fundamental questions aboutone-dimensional systems and are expected to play a central role inapplications ranging from molecular electronics to novel scanningmicroscopy probes. To explore such diverse and exciting opportunitiesrequires nanowire materials for which the chemical composition anddiameter can be varied. Over the past several years considerable efforthas been placed on the bulk synthesis of nanowires, and while advanceshave been made using template, laser ablation, solution, and othermethods, in no case has it been demonstrated that one approach could beexploited in a predictive manner to synthesize a wide range of nanowirematerials. Here we describe the predictable synthesis of a broad rangeof binary and ternary III-V, II-VI and IV-IV group semiconductornanowires using a laser-assisted catalytic growth (LCG) method.

Recently, we reported the growth of elemental Si and Ge nanowires usingthe LCG method, which exploits laser ablation to generate nanometerdiameter catalytic clusters that define the size and direct the growthof the crystalline nanowires by a vapor-liquid-solid (VLS) mechanism. Akey feature of the VLS growth process and our LCG method is thatequilibrium phase diagrams can be used to predict catalysts and growthconditions, and thereby enable rational synthesis of new nanowirematerials. Significantly, we show here that semiconductor nanowires ofthe III-V materials GaAs, GaP, GaAsP, InAs, InP and InAsP, the II-VImaterials ZnS, ZnSe, CdS and CdSe, and IV-IV alloys of SiGe can besynthesized in high yield and purity using this approach. Compoundsemiconductors, such as GaAs and CdSe, are especially intriguing targetssince their direct band gaps give rise to attractive optical andelectrooptical properties. The nanowires have been prepared as singlecrystals with diameters as small as 3 nm, which places them in a regimeof strong radial quantum confinement, and lengths exceeding 10 μm. Thesestudies demonstrate that LCG represents a very general and predictiveapproach for nanowire synthesis, and moreover, we believe that thebroad-range of III-V, II-VI and IV-IV nanowires prepared will open upmany new opportunities in nanoscale research and technology.

The prediction of growth conditions for binary and more complexnanowires using the LCG method is, in principle, significantly moredifficult than previous studies of elemental Si and Ge nanowires due tothe complexity of ternary and higher order phase diagrams. However, thiscomplexity can be greatly reduced by considering pseudobinary phasediagrams for the catalyst and compound semiconductor of interest. Forexample, the pseudobinary phase diagram of Au—GaAs shows that Au—Ga—Asliquid and GaAs solid are the principle phases above 630° C. in the GaAsrich region (FIG. 15). This implies that Au can serve as a catalyst togrow GaAs nanowires by the LCG method, if the target composition andgrowth temperature are set to this region of the phase diagram. Indeed,we find that LCG using (GaAs)_(0.95)Au_(0.05) targets produces samplesconsisting primarily of nanowires. A typical field-emission scanningelectron microscopy (FE-SEM) image of material prepared at 890° C. (FIG.16A) shows that the product is wire-like with lengths extending to 10 μmor more. Analyses of these high-resolution SEM images shows that atleast 90% of the product produced by the LCG method is nanowire withonly a small amount of particle material. X-ray diffraction data frombulk samples can be indexed to the zinc-blende (ZB) structure with alattice constant consistent with bulk GaAs, and also show that thematerial is pure GaAs to the 1% level. Lastly, we note that high yieldsof GaAs nanowires were also obtained using Ag and Cu catalysts. Thesedata are consistent with the fact that these metals (M=Ag, Cu) exhibitM-Ga—As liquid and GaAs solid phase in the GaAs rich regions of thepsuedobinary phase diagrams, and furthermore, demonstrate thepredictability of the LCG approach to nanowire growth.

The structure and composition of the GaAs nanowires have beencharacterized in detail using transmission electron microscopy (TEM),convergent beam electron diffraction (ED) and energy dispersive X-rayfluorescence (EDX). TEM studies show that the nanowires have diametersranging from 3 nm to ca 30 nm. A typical diffraction contrast image of asingle 20 nm diameter wire (FIG. 17A) indicates that the wire is singlecrystal (uniform contrast) and uniform in diameter. The Ga:Ascomposition of this wire determined by EDX, 51.4:48.6, is the same,within limits of instrument sensitivity, as the composition obtainedfrom analysis of a GaAs crystal standard. Moreover, the ED patternrecorded perpendicular to the long axis of this nanowire (inset, FIG.17A) can be indexed for the <112> zone axis of the ZB GaAs structure,and thus shows that growth occurs along the [111] direction. Extensivemeasurements of individual GaAs nanowires show that growth occurs alongthe <111> directions in all cases. This direction and the single crystalstructure are further confirmed by lattice resolved TEM images (e.g.,FIG. 17B) that show clearly the (111) lattice planes (spacing 0.32±0.01nm; bulk GaAs, 0.326 nm) perpendicular to the wire axis. Lastly, the TEMstudies reveal that most nanowires terminate at one end with ananoparticle (inset, FIG. 16A). EDX analysis indicates that thenanoparticles are composed mainly of Au. The presence of Aunanoparticles at the ends of the nanowires is consistent with thepseudobinary phase diagram, and represents strong evidence for a VLSgrowth mechanism proposed for LCG.

The successful synthesis of binary GaAs nanowires by LCG is not anisolated case but general to a broad range of binary and more complexnanowire materials (Table-1). To extend our synthetic approach to thebroadest range of nanowires, we recognize that catalysts for LCG can bechosen in the absence of detailed phase diagrams by identifying metalsin which the nanowire component elements are soluble in the liquid phasebut that do not form solid compounds more stable than the desirednanowire phase; that is, the ideal metal catalyst should be physicallyactive but chemically stable. From this perspective the noble metal Aushould represent a good starting point for many materials. This noblemetal also has been used in the past for the VLS growth of surfacesupported nanowires by metal-organic chemical vapor deposition (MOCVD).The nanowires produced by MOCVD method are distinct from the materialsreported in this communication in several regards, including (1) theMOCVD nanowires are produced on surfaces and not in the bulk quantitiesrequired for assembly, (2) the MOCVD nanowires taper significantly fromthe base to their ends (that is, they do not have uniform diameters),and (3) the smallest nanowire diameters, 10-15 nm, are significantlylarger than the 3-5 nm diameters achieved in our work. Lastly, asdescribed below, it is important to recognize that our LCG method isreadily extended to many different materials (e.g., Table-1) simply byproducing solid targets of the material of interest and catalyst.

First, we have extended significantly our work on GaAs to include GaPand ternary alloys GaAs_(1-x)P_(x). FE-SEM images of the productobtained by LCG from (GaP)_(0.95)Au_(0.05) targets exhibit high puritynanowires with lengths exceeding 10 μm (FIG. 16B). Extensive TEMcharacterization shows that these nanowires (i) are single crystal GaP,(ii) grow along the <111> directions, and (iii) terminate in Aunanoparticles (inset, FIG. 16B) as expected for the LCG mechanism. Wehave further tested the limits of our LCG approach through studies ofternary GaAsP alloy nanowires. The synthesis of ternary III-V alloys isof particular interest for band gap engineering that is critical forelectronic and optical devices. LCG of GaAsP nanowires using aGaAs_(0.6)P_(0.4) target with a Au catalyst yielded nearly purenanowires (FIG. 16C). TEM images, ED and EDX show that these nanowiresare single crystals, grow along the <111> directions, have a Ga:As:Pratio, 1.0:0.58:0.41, that is essentially the same as the startingtarget composition, and terminate in nanoclusters that are composedprimarily of Au (inset, FIG. 16C). High-resolution TEM images recordedon nanowires with diameters of ca.10 and 6 nm (FIGS. 17C and 17D) showwell-ordered (111) lattice planes and no evidence for compositionalmodulation. We believe the observation that the ternary nanowirecomposition can be controlled by target composition is especiallyimportant, because it provides an opportunity to explore exciton energychanges due to both band-gap variations (composition) and quantumconfinement (size).

Based on the above results, it is perhaps not surprising that we havealso successfully used LCG to prepare III-V binary and ternary materialscontaining In—As—P (Table-1). We believe that a more significant pointis that this synthetic approach can also be easily extended to thepreparation of many other classes of nanowires, including the II-VImaterials ZnS, ZnSe, CdS and CdSe (Table-1), IV-IV SiGe alloys. Thecases of the II-VI nanowires CdS and CdSe are especially important,because a stable structural phase of these materials, wurtzite (W), isdistinct from the ZB structure of the III-V materials described aboveand the ZB structure of ZnS and ZnSe. Significantly, we find thatnanowires of CdS and CdSe can be synthesized in high yield using the LCGapproach with a Au catalyst (FIG. 18A). TEM and ED data obtained onindividual CdSe nanowires (for example, FIGS. 18B and 18C) demonstratethat these materials are single crystals with a W-type structure and<110> growth direction that is clearly distinguished from the <111>direction of ZB structures. Studies of CdS nanowires (Table-1) showsomewhat more complex behavior; that is, W-type nanowires with growthalong two distinct <100> and <002> directions. It is possible that the<002> direction assigned for a minority of CdS nanowires couldcorrespond to the <111> direction of a ZB structure. However, X-raydiffraction measurements made on bulk nanowire samples are consistentwith the W assignment. In addition, previous studies of W-type CdS andCdSe nanoclusters showed elongation along the <002> direction. Webelieve that systematic studies of nanowire structure as a function ofgrowth temperature should help to elucidate the origin of these resultsfor CdS, and could also provide insight into how nanowire growthdirection might be controlled.

Lastly, we have used LCG to prepare nanowires of IV-IV binary Si—Gealloys (Table-1). Using a Au catalyst, it was possible to synthesizesingle crystal nanowires over the entire Si_(1-x)Ge_(x) compositionrange. Unlike the case of GaAsP discussed above, the Si—Ge alloys do notexhibit the same compositions as the starting targets. Rather, thecomposition varies continuously within the growth reactor with Si richmaterials produced in the hotter central region and Ge rich materialsproduced at the cooler end. Specifically, LCG growth from a(Si_(0.70)Ge_(0.30))_(0.95)Au_(0.05) target at 1150° C. producednanowires with a Si:Ge ratio of 95:5, 81:19, 74:26, 34:66 and 13:87 fromthe furnace center to end, respectively. This composition variationarises from the fact that the optimal growth temperatures of the twoindividual nanowire materials are quite different. Such differences canincrease the difficulty in synthesizing controlled composition alloys,although our results also show that this can be exploited to prepare arange of alloy compositions in a single growth experiment.

In conclusion, we have synthesized a wide-range of single crystal binaryand ternary compound semiconductor nanowires using our LCG technique. Webelieve that these results demonstrate clearly the generality of thisapproach for rational nanowire synthesis. The availability of thesehigh-quality, single crystal semiconductor nanowires is expected toenable fascinating opportunities in nanometer scale science andtechnology. For example, these nanowires can be used to probe theconfinement, dynamics and transport of excitons in 1D, and can serve asoptically-active building blocks for nanostructured materials. Moreover,by further controlling growth, we believe that our LCG approach can beused to synthesize more complex nanowire structures, including singlewire homo- and heterojunctions and superlattices, and thus may enablethe synthesis of nanoscale light-emitting diodes and laser devices.

The apparatus and general procedures for LCG growth of nanowires havebeen described previously. The targets used in syntheses consisted of(material)_(0.95)Au_(0.05). Typical conditions used for synthesis were(i) 100-500 ton Ar:H₂ (95:5), (ii) 50-150 sscm gas flow, and (iii)ablation with a pulsed Nd:YAG laser (λ=1064 nm; 10 Hz pulse rate; 2.5 Waverage power). Specific temperatures used for the growth of differentnanowire materials are given in Table-1. The nanowire products werecollected at the down-stream cold end of the furnace.

The nanowire samples were characterized using X-ray diffraction (SCINTAGXDS 2000), FE-SEM (LEO 982), and TEM (Philips 420 and JEOL 2010).Electron diffraction and composition analysis (EDX) measurements werealso made on the TEMs. Samples for TEM analysis were prepared asfollows: samples were briefly sonicated in ethanol, which suspended thenanowire material, and then a drop of suspension was placed on a TEMgrid and allowed to dry.

Template mediated methods using membranes and nanotubes have been usedto prepare a number of materials. However, these nanowires typicallyhave diameters >10 nm, which are larger than those desired for strongquantum confinement effects, and often have polycrystalline structuresthat make it difficult to probe intrinsic physical properties.

Table 1 is a summary of single crystal nanowires synthesized. The growthtemperatures correspond to ranges explored in these studies. The minimum(Min.) and average (Ave.) nanowire diameters (Diam.) were determinedfrom TEM and FE-SEM images. Structures were determined using electrondiffraction and lattice resolved TEM imaging: ZB, zinc blende; W,wurtzite; and D, diamond structure types. Compositions were determinedfrom EDX measurements made on individual nanowires. All of the nanowireswere synthesized using Au as the catalyst, except GaAs, for which Ag andCu were also used. The GaAs nanowires obtained with Ag and Cu catalystshave the same size, structure and composition as those obtained with theAu catalyst.

Growth Min. Diam. Ave. Diam. Growth Ratio of Material Temp. (° C.) (nm)(nm) Structure Direction Components GaAs  800-1030 3 19 ZB <111>1.00:0.97 GaP 870-900 3-5 26 ZB <111> 1.00:0.98 GaAs_(0.6)P_(0.4)800-900 4 18 ZB <111> 1.00:0.58:0.41 InP 790-830 3-5 25 ZB <111>1.00:0.98 InAs 700-800 3-5 11 ZB <111> 1.00:1.19 InAs_(0.5)P_(0.5)780-900 3-5 20 ZB <111> 1.00:0.51:0.51 ZnS  990-1050 4-6 30 ZB <111>1.00:1.08 ZnSe 900-950 3-5 19 ZB <111> 1.00:1.01 CdS 790-870 3-5 20 W<100>, 1.00:1.04 <002> CdSe  680-1000 3-5 16 W <110> 1.00:0.99Si_(1−x)Ge_(x)  820-1150 3-5 18 D <111> Si_(1−x)Ge_(x)

FIG. 15 shows a pseudobinary phase diagram for Au and GaAs. The liquidAu—Ga—As component is designated by L.

FIGS. 16A-16C show FE-SEM images of GaAs (FIG. 16A), GaP (FIG. 16B) andGaAs_(0.6)P_(0.4) (FIG. 16C) nanowires prepared by LCG. The scale barsin FIGS. 16A-16C are 2 μm. The insets in FIGS. 16A-16C are TEM images ofGaAs, GaP and GaAs_(0.6)P_(0.4) nanowires, respectively. The scale barsin are all 50 nm. The high contrast (dark) features correspond to thesolidified nanocluster catalysts.

FIG. 17A shows a diffraction contrast TEM image of a ca. 20 nm diameterGaAs nanowire. The inset shows a convergent beam electron diffractionpattern (ED) recorded along the <112> zone axis. The [111] direction ofthe ED pattern is parallel to the wire axis, and thus shows that growthoccurs along the [111] direction. The scale bar corresponds to 20 nm.FIG. 17B shows a high-resolution TEM image of a ca. 20 nm diameter GaAsnanowire. The lattice spacing perpendicular to the nanowire axis,0.32±0.01 nm, is in good agreement with the 0.326 nm spacing of (111)planes in bulk GaAs. The scale bar corresponds to 10 nm. FIGS. 17C and17D show high-resolution TEM images of 10 and 6 nm diameter,respectively, GaAs_(0.6)P_(0.4) nanowires. The (111) lattice planes(perpendicular to the wire axes) are clearly resolved in all threenanowires. The scale bars in FIGS. 17C and 17D are 5 nm.

FIG. 18A shows a FE-SEM image of CdSe nanowires prepared by LCG. Thescale bar corresponds to 2 μm. The inset in FIG. 18A is a TEM image ofan individual CdSe nanowire exhibiting nanocluster (dark feature) at thewire end. EDX shows that the nanocluster is composed primarily of Au.The scale bar is 50 nm. FIG. 18B shows a diffraction contrast TEM imageof a 18 nm diameter CdSe nanowire. The uniform contrast indicates thatthe nanowire is single crystal. The inset in FIG. 18B is an ED pattern,which has been indexed to the wurtzite structure, recorded along the<001> zone axis. The [110] direction of the ED pattern is parallel tothe wire axis, and thus shows that growth occurs along the [110]direction. The scale bar is 50 nm. FIG. 18C shows a high-resolution TEMimage of a ca. 13 nm diameter CdSe nanowire exhibiting well-resolved(100) lattice planes. The experimental lattice spacing, 0.36±0.01 nm isconsistent with the 0.372 nm separation in bulk crystals. The 30°orientation (100) lattice planes with respect to the nanowire axis isconsistent with the [110] growth direction determined by ED. The scalebar corresponds to 5 nm.

Laser-Assisted Catalytic Growth of Single Crystal GaN Nanowires

Single crystalline GaN nanowires have been synthesized in bulkquantities using laser-assisted catalytic growth (LCG). Laser ablationof a (GaN, Fe) composite target generates liquid nanoclusters that serveas catalytic sites confining and directing the growth of crystallinenanowires. Field emission scanning electron microscopy shows that theproduct primarily consists of wire-like structures, with diameters onthe order of 10 nm, and lengths greatly exceeding 1 Powder X-raydiffraction analyses of bulk nanowire samples can be indexed to the GaNwurtzite structure, and indicate >95% phase purity. Transmissionelectron microscopy, convergent beam electron diffraction, and energydispersive X-ray fluorescence analyses of individual nanowires show thatthey are GaN single crystals with a [100] growth direction. Thesynthesis of bulk quantities of single crystal nanowires of GaN andother technologically important semiconducting nitride materials shouldopen up many opportunities for further fundamental studies andapplications.

Herein we report the bulk synthesis of single crystalline GaN nanowires.Laser ablation of a composite target of GaN and a catalytic metalgenerates liquid nanoclusters that serve as reactive sites confining anddirecting the growth of crystalline nanowires. Field emission scanningelectron microscopy (FE-SEM) shows that the product primarily consistsof wire-like structures. Powder X-ray diffraction (PXRD) analyses ofbulk nanowire sample can be indexed to the GaN wurtzite structure, andindicate >95% phase purity. Transmission electron microscopy (TEM),convergent beam electron diffraction (CBED), and energy dispersive X-rayfluorescence (EDX) analyses of individual nanowires show that they areGaN single crystals with a [100] growth direction.

Nanostructured GaN materials have attracted extensive interest over thepast decade due to their significant potential for optoelectronics.These studies have primarily focused on zero dimensional (OD) quantumdots and two dimensional (2D) quantum well structures, which can bereadily synthesized using established methods. Investigations of onedimensional (1D) GaN nanowires, which could enable unique opportunitiesin fundamental and applied research, have been limited due todifficulties associated with their synthesis. Specifically, there hasbeen only one report of GaN nanowire growth. In this work, carbonnanotubes were used as templates in the presence of Ga-oxide and NH₃vapor to yield GaN nanowires. We have exploited predictable syntheticapproach for GaN nanowire growth called laser-assisted catalytic growth(LCG). In this method, a pulsed laser is used to vaporize a solid targetcontaining desired material and a catalyst, and the resulting liquidnanoclusters formed at elevated temperature direct the growth and definethe diameter of crystalline nanowires through a vapor-liquid-solidgrowth mechanism. A key feature of this method is that the catalyst usedto define 1D growth can be selected from phase diagram data and/orknowledge of chemical reactivity. A related approach termedsolution-liquid-solid phase growth has been used by Buhro and coworkersto prepare nanowires of several III-V materials in solution, althoughnot nitrides.

In the case of GaN, detailed information on ternary phase diagramsrelevant to LCG (i.e., catalyst-Ga—N) is unavailable. However, we canuse knowledge of the growth process to choose rationally a catalyst.Specifically, the catalyst should form a miscible liquid phase with GaNbut not form a more stable solid phase under the nanowire growthconditions. The guiding principle suggests that Fe, which dissolves bothGa and N, and does not form a more stable compound than GaN will be agood catalyst for GaN nanowire growth by LCG. The overall evolution ofnanowire growth following the generation of the catalytic nanocluster bylaser ablation is illustrated in FIG. 19.

Significantly, we find that LCG using a GaN/Fe target produces a highyield of nanometer diameter wire-like structures. A typical FE-SEM imageof the product produced by LCG (FIG. 20A) shows that the productconsists primarily of 1D structures with diameters on the orders of 10nm and lengths greatly exceeding 1 μm; that is, high aspect rationanowires. The FE-SEM data also show that the products consist of ca.90% nanowires, with the remaining being nanoparticles. We have alsoassessed the overall crystal structure and phase purity of the bulknanowire samples using PXRD (FIG. 20B). All the relatively sharpdiffraction peaks in the PXRD pattern can be indexed to a wurtzitestructure with lattice constants of a=3.187 and c=5.178 Å. These valuesare in good agreement with literature values for bulk GaN: a=3.189,c=5.182 Å. In addition, comparison of the background signal and observedpeaks indicates that the GaN wurtzite phase represents >95% of thecrystalline material produced in our syntheses.

The LCG experimental apparatus is similar to that reported previously. AGaN/Fe (atomic ratio (GaN):Fe=0.95:0.05) composite target was positionedwith a quartz tube at the center of a furnace. The experimental systemwas evacuated to 30 mtorr, and then refilled with anhydrous ammonia gas.While the pressure and flow rate were maintained at ca. 250 torr and 80sccm, respectively, the furnace temperature was increased to 900° C. at30° C./min. A pulsed Nd-YAG laser (1064 nm, 8 ns pulse width, 10 Hzrepetition, 2.5 W average power) was then used to ablate the target witha typical ablation duration of 5 min. After ablation, the furnace wasturned off and allowed to cool to room temperature. The system was thenvented and light yellowish powders were collected from the end of innerquartz tube wall. The product was used directly for FE-SEM and PXRDstudies. The product was suspended in ethanol and then transferred ontoTEM grids for TEM, CBED and EDX measurements.

The morphology, structure and composition of the GaN nanowires have beencharacterized in further detail using TEM, CBED and EDX. TEM studiesshow that the nanowires are straight with uniform diameters, andtypically terminate in a nanoparticle at one end. FIG. 20A shows arepresentative diffraction contrast image of one nanowire. The uniformcontrast along the wire axis indicates that the nanowire is a singlecrystal. The nanoparticle (dark, high contrast feature) observed at thenanowire end is faceted as expected following crystallization of theliquid nanocluster (FIG. 19). We have also used EDX to address thecomposition of the nanowires and terminal nanoparticles. Data recordedon the nanowire show only Ga and N in a ratio ca. the same as a GaNstandard, while the nanoparticles contain Ga, N, and Fe. The presence ofFe (with Ga and N) only in the terminal nanoparticle confirms thecatalytic nature of Fe in the synthesis.

To probe further the importance of the catalyst, we have alsoinvestigated GaN nanowire growth using a Au catalyst. Gold has been usedrecently as a catalyst for growth of a number of nanowires of III-V andII-VI material, and as such might be expected to also functioneffectively in the growth of GaN nanowires. However, Au exhibits poorsolubility of N and thus may not transport N efficiently to theliquid/solid growth interface. Consistent with this analysis, we havebeen unable to obtain GaN nanowire using the Au catalyst. We believethat this highlights the important role of the catalyst and how it canbe rationally chosen.

Lastly, we have characterized the structure of GaN nanowires in greaterdetail using CBED and high resolution TEM (HRTEM). A typical CBEDpattern (inset, FIG. 21A) of a nanowire exhibits a sharp diffractionpattern consistent with the single crystal structure inferred from thediffraction contrast images. Indexing this pattern further demonstratesthat the [100] direction is aligned along the wire axis. In addition,FIG. 21B shows a lattice resolved HRTEM image of a GaN nanowire with aca. 10 nm diameter. The image, which was recorded along the <001> zoneaxis, shows clearly the single crystal structure of the nanowire and thelattice planes along the [100], [010] and [−110] directions. This imagedemonstrates that the [100] direction runs parallel to the wire axis,and thus confirms the [100] growth direction in GaN nanowires.

In conclusion, we have exploited the LCG method for the rationalsynthesis of GaN nanowires. Highly pure GaN nanowires were obtained assingle crystals with a unique [100] growth direction. We believe thatthis approach, which is based on the predictable choice of catalyst andgrowth conditions, can be readily extended to the synthesis of InN,(GaIn)N alloys and related nitride nanowires. The synthesis of bulkquantities of single crystal nanowires of GaN and other technologicallyimportant semiconducting nitride materials is expected to open up manyopportunities for further fundamental studies and applications.

FIG. 19 is a schematic diagram showing GaN nanowire growth bylaser-assisted catalytic growth.

FIG. 20A shows a FE-SEM (LEO 982) image of bulk GaN nanowiressynthesized by LCG. The scale bar corresponds to 1 μm. FIG. 20B shows aPXRD (Scintag, XDS2000) pattern recorded on bulk GaN nanowires. Thenumbers above the peaks correspond to the (hkl) values of the wurtzitestructure.

FIG. 21A shows a diffraction contrast TEM (Philips, EM420) image of aGaN nanowire that terminates in a faceted nanoparticle of higher(darker) contrast. The inset in FIG. 21A shows a CBED pattern recordedalong <001> zone axis over the region indicated by the white circle. Thewhite scale bar corresponds to 50 nm. FIG. 21B shows a HRTEM (JEOL 2010)image of another GaN nanowire with a diameter of ca. 10 nm. The imagewas taken along <001> zone axis. The [100], [010] and [−110] directionsare indicated with the [100] parallel to the wire axis. The white scalebar corresponds to 5 nm.

Nanoscale Electronic and Optoelectronic Devices Assembled from IndiumPhosphide Nanowire Building Blocks

One dimensional nanostructures, such as nanowires (NWs) and nanotubes(NTs), are ideally suited for efficient transport of charge carriers andexcitons, and thus are expected to be critical building blocks fornanoscale electronics and optoelectonics. Studies of electricaltransport in carbon NTs have led to the creation of field effecttransistors (FETs), single electron transistors, rectifying junctionsand chemical sensors. These results indicate exciting applicationspossible from such materials, although the use of NT building blocks isquite limited in that selective growth and/or assembly of semiconductingor metallic NTs is not currently possible. The use of nanoscalestructures as building blocks for bottom-up assembly of active devicesand device arrays, which can eliminate the need for costly fabricationlines, will require that the electronic properties of the differentblocks be both defined and controllable. To this end we report therational assembly of functional nanoscale devices from compoundsemiconductor NW building blocks in which the electrical properties havebeen controlled by doping. Gate-dependent transport measurementsdemonstrate that indium phosphide (InP) NWs can be synthesized withcontrolled n-type and p-type doping, and can function as nanoscale FETs.In addition, the availability of well-defined n- and p-type materialshas enabled the creation of p-n junctions by forming crossed NW arrays.Transport measurements reveal that the nanoscale p-n junctions exhibitwell-defined current rectification. Significantly, forward biased InPp-n junctions exhibit strong, quantum confined light emission makingthese structures perhaps the smallest light emitting diodes created todate. Lastly, electric field directed assembly is shown to be onestrategy capable of creating highly integrated and functional devicesfrom these new nanoscale building blocks.

Single crystal InP NWs have been prepared by a laser-assisted catalyticgrowth (LCG), which has been described previously. The n-type and p-typeInP NWs were prepared using tellurium (Te) and zinc (Zn) as dopants,respectively, and found to be of similar high quality as NWs producedwithout the addition of dopants. Field emission scanning electronmicroscopy (FE-SEM) images of as-synthesized Zn-doped InP NWs (FIG. 22A)demonstrate that the wires extend up to tens of micrometers in lengthwith diameters on the order of 10 nanometers. High-resolutiontransmission electron microscopy (TEM) images (inset, FIG. 22A) furthershow that the doped NWs are single crystals with <111> growthdirections. Generally, a 1-2 nm amorphous over-layer on the NWs isvisible in TEM images. This thin layer is attributed to oxides formedwhen the NWs are exposed to air after synthesis. The overall compositionof individual NWs determined by energy dispersive X-ray (EDX) analysiswas found to be 1:1 In:P, thus confirming the stoichiometric compositionof the NWs. EDX and other elemental analytic methods are, however,insufficiently sensitive to determine the doping level in individualNWs.

To confirm the presence and type of dopants in the NWs, we haveperformed gate-dependent, two terminal transport measurements onindividual NWs. In these measurements, the NW conductance will respondin an opposite way to change in gate voltage (V_(g)) for n-and p-typeNWs. Specifically, V_(g)>0 will lead to an accumulation of electrons andan increase in conductance for n-type NWs, while the same applied gatewill deplete holes and reduce conductance for p-type NWs. FIGS. 22B and22C and 100 c show the typical gate-dependent I-V curves obtained fromindividual Te- and Zn-doped NWs respectively. The I-V curves are nearlylinear for both types of NWs at V_(g)=0, indicating the metal electrodesmake ohmic contact to the NWs. The transport data (FIG. 22B) recorded onTe-doped NWs show an increase in conductance for V_(g)>0, while theconductance decreases for V_(g)<0. These data clearly show that Te-dopedInP NWs are n-type. Gate-dependent transport data recorded on Zn-dopedNWs show opposite changes in conductance with variation in V_(g)compared to the n-type, Te-doped InP NWs. Specifically, for V_(g)>0,conductance decreases and for V_(g)<0 conductance increases (FIG. 22C).These results demonstrate that the Zn-doped InP NWs are p-type.

Our results are quite reproducible. Measurements made on over twentyindividual NWs, with diameters ranging from 20 nm to 100 nm, show gateeffects in each case that are consistent with the dopant used during InPNW synthesis. In addition, the gate voltage can be used to completelydeplete electrons and holes in n- and p-type NWs such that theconductance becomes immeasurably small. For example, the conductance ofthe NW in FIG. 22B can be switched from a conducting (on) to aninsulating (off) state when V_(g) is less than or equal to −20 V, andthus it functions as a FET. The conductance modulation can be as largeas 4-5 orders of magnitude for some of the NWs. The relatively largeswitching voltage is related to the thick (600 nm) oxide barrier used inour measurements. This gate-dependent behavior is similar to that ofmetal-oxide-semiconductor (MOS) FETs and recent studies ofsemiconducting NT FETs. An important distinction of our work withrespect to NTs is that predictable semiconducting behavior can beachieved in every NW. Taken together, these results clearly illustratethat single crystal InP NWs can be synthesized with controlled carriertype. Because these NWs are produced in bulk quantities, they representa readily available material for assembling devices and device arrays.

The availability of well-defined n-and p-type NW building blocks opensup the possibility of creating complex functional devices by formingjunctions between two or more wires. To explore this excitingopportunity, we have studied the transport behavior of n-n, p-p and p-njunctions formed by crossing two n-type, two p-type, and one n-type andone p-type NW, respectively. FIG. 23A shows a representative crossed NWdevice formed with a 29 nm and 40 nm diameter NW. The four arms aredesignated as A, B, C, D for the simplicity of discussion below.Significantly, the types of junctions studied are controllable for everyexperiment since we can select the types of NWs used to produce thecrossed junction prior to assembly.

FIGS. 23B and 23C show the current-voltage (I-V) data recorded on n-nand p-p junctions, respectively. For both types of junctions, thetransport data recorded on the individual NWs (AC, BD) show linear ornearly linear I-V behavior (curves 80, FIG. 23B and curve 82, FIG. 23C).These results show that the metal electrodes used in the experimentsmake ohmic or nearly ohmic contact to the NWs, and will not makenonlinear contributions to the I-V measurements across junctions. Ingeneral, transport measurements made across the n-n and p-p junctionsshow linear or nearly linear behavior, and allow us to infer twoimportant points about junctions made in this way. First, interfaceoxide between individual NWs does not produce a significant tunnelingbarrier, since such a barrier will lead to highly non-linear I-Vbehavior. Second, the I-V curves recorded through each pair (AB, AD, CB,CD) of adjacent arms shows a similar current level, which is smallerthan that of the individual NWs themselves. These results demonstratethat the junction dominates the transport behavior. Lastly, our dataindicate that individual NWs make reasonably good electrical contactwith each other, despite the small contact area (10⁻¹²-10⁻¹⁰ cm²) andsimple method of junction fabrication.

The good contact between individual NWs provides the basis for exploringthese NW to make functional devices. As an example, we have made p-njunctions from crossed p- and n-type NWs. These junctions can be madereproducibly by sequential deposition of dilute solutions of n- andp-type NWs with intermediate drying. FIG. 23D shows typical I-V behaviorof a crossed NW p-n junction. The linear I-V of the individual n- andp-type NWs components (curves 84 and 86) indicates ohmic contact betweenthe NWs and metal electrodes. Transport behavior across the p-n junction(curves 88) shows clear current rectification; i.e., little currentflows in reverse bias, while there is a sharp current onset in forwardbias. Significantly, the behavior is similar to bulk semiconductor p-njunctions, which form the basis for many critical electronic andoptoelectronic devices. In a standard p-n junction, rectification arisesfrom the potential barrier formed at the interface between p- and n-typematerials. When the junction is forward biased (p-side positivelybiased), the barrier is reduced and a relatively large current can flowthrough the junction; on the other hand, only small current can flow inreverse bias since the barrier is further increased.

There are several reasons we believe that the observed rectification isdue to the p-n junction formed at the crossing point between p- andn-type InP NWs. First, the linear or nearly linear I-V behavior ofindividual p- and n-type NWs used to make the junction shows that ohmiccontact have been made between the NWs and metal electrodes. Thisexcludes the possibility that rectification arises frommetal-semiconductor Schottky diodes. Second, the I-V behavior of thejunction determined through every pair (AB, AD, CD, CD) of adjacentelectrodes (curves 88 in FIG. 23D) exhibits a similar rectificationeffect and current level, which is also much smaller than the currentlevel through the individual NWs. These results demonstrate that thejunction dominates the I-V behavior. Third, four terminal measurementsin which current is passed through two adjacent electrodes (e.g., A-B)while the junction voltage drop is measured across two independentelectrodes (e.g., C-D) exhibit similar I-V and rectification with only aslightly smaller voltage drop (0.1-0.2V) compared to two terminalmeasurements at the same current level. Lastly, measurements made on tenindependent p-n junctions showed similar rectification in the I-V data;i.e., significant current can only flow through p-n junctions when thep-type NW is positively biased.

The above data show unambiguously that we can now rationally fabricatenanoscale p-n junctions. In direct band gap semiconductors like InP, thep-n junction forms the basis for the critical optoelectronics devices,including light emitting diodes (LED) and lasers. To assess whether ournanoscale devices might behavior similarly, we have studied thephotoluminescence (PL) and electroluminescence (EL) from crossed NW p-njunctions. Significantly, EL can be readily observed from thesenanoscale junctions in forward bias. FIG. 24A shows an EL image takenfrom a typical NW p-n junction at forward bias, and the inset shows thePL image of a crossed NW junction. The PL image clearly shows twoelongated wire-like structures, and the EL image shows that the lightcomes from a point-like source. Comparison of the EL and PL images showsthat the position of the EL maximum corresponds to the crossing point inthe PL image, thus demonstrating the light indeed comes out from the NWp-n junction.

The I-V characteristic of the junction (inset, FIG. 24B) shows clearrectification with a sharp current onset at ˜1.5 volts. The EL intensityversus voltage curve of the junction shows significant light can bedetected with our system at a voltage as low as 1.7 volts. The ELintensity increases rapidly with the bias voltage, and resembles the I-Vbehavior. The EL spectrum (FIG. 24C) shows a maximum intensity around820 nm, which is significantly blue shifted relative to the bulk bandgap of InP (925 nm). The blue-shift is due in part to quantumconfinement of the excitons, although other factors may also contribute.The importance of quantum confinement can be seen clearly in EL resultsrecorded from p-n junctions assembled from smaller (and larger) diameterNWs (FIG. 24D), which show larger (smaller) blue-shifts. The ability totune color with size in these nanoLEDs might be especially useful in thefuture. The quantum efficiency (electron to photon) of these initialdevices is relatively low, ˜0.001%, which is not surprising since wehave paid little attention to optimization. The efficiency is actuallycomparable to that (˜0.002%) of early bulk InP LEDs. We attribute thelow quantum efficiency to non-radiative recombination via surfacestates, and believe that this deleterious process can be reduced bysurface passivation.

GaN is a direct wide bandgap semiconductor material, which emits lightin the short wavelength (UV and blue) region at room temperature. BlueLEDs are important as emitters where strong, energy efficient andreliable light source are needed. Also it is important to enableproduction of full color LED displays and LED white lamp, since blue isone of the three primary colors (red, green and blue).

Here we report the first made BLUE/UV nanoLEDs (light emitting region onthe order of 10 nm's), which is constructed with P-type Si and N-type(unintentionally doped)

GaN nanowires. Together with the nanoLED we reported before which emitslight in the near IR region, we show the great potential of making LEDswith different materials that would cover the full color spectrum.

FIG. 25A shows an EL image taken from two P-type Si and N-type GaNcrossed nanojunctions. The p-Si is doped with Boron. FIG. 25B showscurrent vs. voltage for various gate voltages. The nanojunction showsgood rectification at different gate voltages. The El spectrum shown inFIG. 25C shows light emission is about 380 nm and 470 nm. A n-InP andp-Si nanojunction has good rectification.

To make highly integrated NW-based devices will ultimately requiretechniques to align and assemble these building blocks into well-definedarrays. To demonstrate the viability of this next stage of development,we have used electric fields (E-field) to align and position individualNWs into parallel and crossed arrays-two basic geometries forintegration. The E-field directed assembly was carried out by placing asolution of NWs between electrodes (FIG. 26A), and then applying a biasof 50-100 V. The potential of this approach is readily seen in the caseof alignment of a chlorobenzene suspended NWs between parallelelectrodes (FIG. 26B). FE-SEM images show that nearly all of the NWs arealigned perpendicular to the parallel electrodes and along E-fielddirection. We have also used electrode arrays to position individual NWsat specific positions. For example, E-field assembly of NWs between anarray of electrodes (FIG. 26C) demonstrates that individual NWs can bepositioned to bridge pairs of diametrically-opposed electrodes and forma parallel array. In addition, by changing the field direction, thealignment can be done in a layer-by-layer fashion to produce crossed NWjunctions (FIG. 26D). These data clearly show that E-field assemblyrepresents a strategy to rationally deposit individual NWs with highdegrees of directional and spatial control. We believe that highlyintegrated functional devices will be readily accessible using our NWbuilding blocks in conjunction with this E-field and/or other assemblytechniques.

Taken as a whole, the results presented in this letter provide arational approach for the bottom-up assembly of nanoscale electronic andoptoelectronic devices. Our demonstrated ability to assemble activedevices in the absence of multi-billion dollar fabrication lines is ofcritical importance to the field and we believe augers well for theimmediate and longer-term advances. We believe that the broad range ofNW materials now available and the clearly defined ability to controltheir electronic properties will make possible nanoscale LEDs that coverthe entire visible and near infrared range (e.g., GaN NWs for bluecolor). Such nanoscale light sources might be useful in creating newtypes of highly parallel optical sensors and for optical inter-connectsin nanoelectronics. Moreover, the assembly of doped NW building blocksclearly has great potential for creating many other types of electronicdevices and possibly even lasers.

InP NWs were synthesized using LCG. The LCG target typically consistedof 94% (atomic ratio) InP, 5% Au as the catalyst, and 1% of Te or Zn asthe doping element. The furnace temperature (middle) was set at 800° C.during growth, and the target was placed at the upstream end rather thanmiddle of the furnace. A pulsed (8 ns, 10 Hz) Nd-YAG laser (1064 nm) wasused to vaporize the target. Typically, growth was carried out for 10minutes with NWs collected at the downstream, cool end of the furnace.

Transport measurement on individual NWs were carried out using publishedprocedures. Briefly, NWs were first dispersed in ethanol, and thendeposited onto oxidized silicon substrates (600 nm oxide, 1-10 Ω·cmresistivity), with the conductive silicon used as a back gate.Electrical contact to the NWs was defined using electron beamlithography (JEOL 6400). Ni/In/Au contact electrodes were thermallyevaporated. Electrical transport measurements were made using home builtsystem with <1 pA noise under computer control. The n-n and p-pjunctions were obtained by random deposition. We first deposited

NWs onto oxidized silicon substrates using relatively highconcentrations, determined the positions of crossed NWs, and thendefined electrodes on all four arms of the cross by electron beamlithography. Ni/In/Au electrodes were used to make contact to the NWs.

The p-n junctions were obtained by layer-by-layer deposition. First, adilute solution of one type (e.g., n-type) of NW was deposited on thesubstrate, and the position of individual NWs was recorded. In a secondstep, a dilute solution of the other type (e.g., p-type) of NW wasdeposited, and the positions of crossed n- and p-type NWs were recorded.Metal electrodes were then defined and transport behavior was measured.

EL was studied with a home-built micro-luminescence instrument. PL orscattered light (514 nm, Ar-ion laser) was used to locate the positionof the junction. When the junction was located, the excitation laser wasshut off, and then the junction was forward biased. EL images were takenwith a liquid nitrogen cooled CCD camera, and EL spectra were obtainedby dispersing EL with a 150 line/mm grating in a 300 mm spectrometer.

FIGS. 22A-22C illustrate doping and electrical transport of InP NWs.FIG. 22A shows a typical FE-SEM image of Zn-doped InP NWs. Scale bar is10 μm. inset, lattice resolved TEM image of one 26 nm diameter NW. The(111) lattice planes are visible perpendicular to the wire axis. Scalebar is 10 nm. FIGS. 22B and 22C show gate-dependent I-V behavior for Te-and Zn-doped NWs, respectively. The insets in FIGS. 22B and 22C show theNW measured with two terminal Ni/In/Au contact electrodes. The scalebars correspond to 1 μm. The diameter of the NW in FIG. 22B is 47 nm,while that in FIG. 22C is 45 nm. Specific gate-voltages used in themeasurements are indicated on the right hand sides of the Figs. on thecorresponding I-V curves. Data were recorded at room temperature.

FIGS. 23A-23D illustrate crossed NW junctions and electrical properties.FIG. 23A shows a FE-SEM image of a typical crossed NW device withNi/In/Au contact electrodes. The scale bar corresponds to 2 μm. Thediameters of the NWs are 29 nm (A-C) and 40 nm (B-D); the diameters ofthe NWs used to make devices were in the range of 20-75 nm. FIGS.23B-23D show I-V behavior of n-n, p-p and p-n junctions, respectively.The curves 80 and 82 correspond to the I-V behavior of individual n- andp-NWs in the junctions, respectively. The curves 88 represent the I-Vbehavior across the junctions. The current recorded for the p- andn-type NWs in FIG. 23D is divided by 10 for better viewing. The solidlines represent transport behavior across one pair of adjacent arms, andthe dashed lines represent that of the other three pairs of adjacentarms. Data were recorded at room temperature.

FIGS. 24A-24D illustrate optoelectrical characterization of NW p-njunctions. FIG. 24A is an EL image of the light emitted from a forwardbiased NW p-n junction at 2.5 V. The inset in FIG. 24A shows the PLimage of the junction. Both scale bars correspond to 5 μm. FIG. 24Bshows the EL intensity versus voltage. The inset in FIG. 24B shows theI-V characteristics and the inset in the inset shows the FE-SEM image ofthe junction itself. The scale bar corresponds to 5 μm. The n-type andp-type NWs forming this junction have diameters of 65 and 68 nm,respectively. FIG. 24C shows an EL spectrum of the junction shown inFIG. 24A. The spectrum peaks at 820 nm. FIG. 24D shows an EL spectrumrecorded from a second forward biased crossed NW p-n junction. The ELmaximum occurs at 680 nm. The inset in FIG. 24D shows the EL image anddemonstrates that the EL originates from the junction region. The scalebar is 5 μm. The n-type and p-type NWs forming this junction havediameters of 39 and 49 nm, respectively.

FIGS. 26A-26D illustrate parallel and orthogonal assembly of NWs withE-fields. FIG. 26A is a schematic view of E-field alignment. Theelectrodes (orange) are biased at 50-100 V after a drop of NW solutionis deposited on the substrate (blue) FIG. 26B shows a parallel array ofNWs aligned between two parallel electrodes. The NWs were suspended inchlorobenzene and aligned using an applied bias of 100 V. FIG. 26C showsa spatially positioned parallel array of NWs obtained following E-fieldassembly using a bias of 80 V. The top inset in FIG. 26C shows 15 pairsof parallel electrodes with individual NWs bridging each diametricallyopposed electrode pair. FIG. 26D shows a crossed NW junction obtainedusing layer-by-layer alignment with the E-field applied in orthogonaldirections in the two assembly steps. The applied bias in both steps was80 V. The scale bars in FIGS. 26B-26D correspond to 10 μm.

Bottom-Up Assembly of Nanoscale Electronic Devices from SiliconNanowires

Four types of important functional nanodevices have been created byrational bottom-up assembly from p and n-type silicon nanowires (SiNWs)with well controlled dopant type and level. In all these devices,electrical transport measurements on individual p and n-type SiNWssuggested ohmic or nearly ohmic contact between SiNWs and leads.Significantly, four-probe measurements across pn junctions consisting ofcrossed p-type and n-type SiNWs showed current rectification behavior asexpected for pn diode behavior. n⁺pn crossed junctions were alsoassembled to create bipolar transistors, in which common base/emittercurrent gains as large as 0.94/16 were obtained. Complementary invertersmade of crossed lightly doped pn junctions showed clear output voltageinverse to input voltage with a gain of 0.13. Tunnel diodes in form ofheavily doped SiNW pn crosses showed negative differential resistance(NDR) behavior in forward bias with a peak-to-valley ratio (PVR) of 5 to1.

Miniaturization of conventional electronics has been intensely pursuedrecently. But the fundamental limits of lithographical methods willprevent the current techniques from reaching the deep nanoelectronicsregime. The use of nanoscale structures as building blocks for thebottom-up assembly of integrated devices, where both the fabrication andassembly of individual blocks are expected to be cheap, can thuseliminate greatly the cost of fabrication lines while still maintainingsome concepts that have proven successful in microelectronics. Onedimensional structures such as nanowires (NWs) and nanotubes (NTs) areideal candidates as critical building blocks for nanoelectronics. How toconstruct the functional nanodevices and device arrays with thesebuilding blocks is essential to nano science and technology. NTs havebeen tested as field effect transistors, single electron transistors.The NT-NW heterojunctions, NT intramolecular junctions and crossedjunctions have also been demonstrated. However, the use of NTs inrational assembly is limited by unpredictability of individual tubeproperties because the specific growth of metallic and semiconductor NTsis not controllable and controlled doping of semiconductor NTs isdifficult.

Previously, we demonstrated the controlled doping of single crystalsemiconductor SiNWs, where the type of dopant (p-type and n-type) andthe relative doping concentration (from lightly to degenerately) werewell controlled. These SiNWs, whose properties are predictable andcontrollable, therefore provide the critical building blocks forbottom-up assembly of active devices and device arrays. It is possiblethat the highly dense SiNW device arrays can be formed by the directedassembly of chemical assembly, for example, the specific peptide bindingto semiconductor, DNA base matching interaction, and/or theligand-receptor interaction. To realize workable integrated devices,understanding the electrical properties of individual bottom-upassembled active devices is the prerequisite. Here we report therational assembly of functional nanodevices from these SiNWs withdiameters from 20 to 50 nm and the device electrical properties. And wedemonstrate that control of dopant type and doping level provides us thecapability to fabricate multiple types of electronic devices. Four typesof important functional structures including pn diodes, bipolartransistors, complementary inverters and tunnel diodes were created bycontrollably combining SiNWs of varying p and n-type doping levels.Nanoscale pn junctions were created in form of crossed SiNW junctions.Electrical transport measurements on these pn junctions showed thecurrent rectification predictable by semiconductor physics. We haveexploited our ability to construct n⁺pn crossed SiNW junctions tobipolar transistors which were demonstrated to have common base/emittercurrent gains as large as 0.94/16. The inverters made of lightly dopedpn crosses showed clearly the output voltage inverse to the inputvoltage with voltage gain of 0.13. And the results of tunnel diodes madeof heavily doped pn crossed showed NDR behavior in forward bias with aPVR of 5 to 1. The p-type and n-type SiNWs were synthesized by usingdiborane and phosphorus, respectively as doping source duringlaser-assisted catalytic growth of SiNWs. Metal leads contact with SiNWson doped silicon substrate with 600 nm thermal oxide were defined byelectron beam lithography. The pn, pp and nn junctions were formed bycrossing one p-type and one n-type, two p-type and two n-type SiNWs,respectively. The types of junctions were controlled by choosing thetypes of SiNWs used to create a given junction. A typical field emissionscanning electron microscopy (FE-SEM) image of cross junctions is shownin FIG. 27A, where the four contact leads are labeled as 1, 2, 3 and 4for the convenience of discussion. FIG. 27B shows current versus voltage(I-V) data on a pn crossed junction with diameters of p and n-type SiNWsas small as 20.3 nm and 22.5 nm, respectively. Four-terminalmeasurements across junction were performed by flowing current betweentwo adjacent leads (e.g., leads 1-2 or leads 1-4, the positive currentdirection is from p to n-type SiNW) and measuring the voltage dropbetween the other two leads (e.g., leads 3-4 or leads 3-2). The I-Vcurve across junction (FIG. 27B curve 130) shows little current inreverse bias (negative bias in our setup) and very sharp current onsetin forward bias (positive bias). In contrast, single p (between leads1-3) and n-type (between leads 2-4) SiNWs show linear I-V behavior (FIG.27B curves 110 and 120, respectively), which suggests ohmic (notrectifying) contact between SiNWs and leads. And thus this rectifyingbehavior must be caused by junction itself. This behavior can beexplained by the energy band diagrams of a pn junction diode. Thebuilt-in potential barrier forms at the junction interface when p andn-type SiNW contact with each other. Electrons can not tunnel throughthe wide space charge region forming at the junction interface but canbe transported by thermal excitation. Forward bias decreases thebuilt-in potential barrier and thus large amount of current can flow(FIG. 27E), while reverse bias increases the barrier and thus currentlevel is low (FIG. 27F).

The p and n-type SiNWs were dispersed in to aceton separately. p-njunctions were obtained by sequential deposition. The solution of onetype of SiNWs (e.g., n-type) was first deposited onto the substrate andthe positions of SiNWs were recorded with respect to alignment marks.Secondly, the solution of the other type of SiNWs (e.g., p-type) wasdeposited and the positions of crossed pn junctions were recorded. pp ornn junctions were obtained by depositing only one type of SiNWs: p-typeor n-type. The junction positions were then recorded.

The reasons why we believe the rectifying behavior is pn diode behaviorinstead of some other asymmetric tunneling barrier at the junctioninterface are: (a). The intrinsic oxide layer of SiNWs is thin enoughthat electrons can easily tunnel through the oxide layer and thereasonable strong coupling between p and n-type wire at the junctionstill exists and thus the built-in potential barrier can form. This isconfirmed by the transport measurements on pp and nn junctions. Thesingle wires (between leads 1-3, 2-4) in pp (FIG. 27C curves 110) and nnjunctions (FIG. 27D curves 120) show linear or almost linear I-Vbehavior suggesting good contact. Two terminal measurements (betweenleads 1-2, 1-4, 2-3, or 3-4) on pp (FIG. 27C curves 130) and nn (FIG.27D curves 130) junctions show linear and almost linear I-V. Comparingtwo-terminal measurement resistance across junctions to single SiNWresistance, we find that the magnitude of junction resistance is similarto the wire resistance, suggesting that the oxide doesn't causesignificant electron tunneling barrier. (b) The measurements on 20independent pn junctions showed consistent correct rectifying behavior.

As the basic unit of most semiconductor devices, pn junctions providethe characteristics needed for rectifiers, amplifiers, switchingcircuits and many other electronic circuit functions. Success in makingpn junction from SiNW crosses provides us the possibility to make otherimportant functional devices. To demonstrate, we can create not only apassive device: p-n diode, but also the active device; we constructedbipolar transistor, which is capable of current gain. A bipolartransistor is a n⁺pn (FIG. 28A left) or p⁺np junction device, whichrequires high doping level in emitter, low doping in base and collector.Well control in doping of SiNWs provides us the capability to make thiscomplex device. Our n⁺pn bipolar transistors were constructed bymechanically manipulating two n-type SiNWs (one heavily doped, the otherlightly doped) onto one lightly doped p-type wire and were operated incommon base configuration (FIG. 28A right). FIG. 28B is a typical SEMimage of bipolar transistors. The SiNWs and junctions in transistorswere first characterized individually. The I-V curves of threeindividual SiNWs are linear and the two individual junctions havecorrect rectifying behavior. Then the n⁺-type SiNW was used as emitterwhile the n-type as collector to do bipolar transistor measurements. Theemitter-base (E-B) is always forward biased to inject electrons intobase region. When the collector-base (C-B) voltage is greater than zero,the transistor is operated in the active mode, in which the C-B junctionis reverse biased and only a very small leakage current will flow acrossthe junction. However, the electrons injected from emitter can diffusethrough the base to reach the C-B junction space charge region and willbe collected by collector. The actual collector current depends only onthe injected electrons from emitter and thus depends only on the E-Bvoltage. This is clearly seen in FIG. 28C regime II, where the collectorcurrent goes high with the forward E-B voltage while change slowly withC-B voltage which results from Early effect and the existence of slowlyincreasing leakage current with reverse bias. This demonstrates thetransistor action: large current flow in a reverse biased collectorjunction can result from carriers injected from a nearby emitterjunction. When the (C-B) voltage is bellow zero, the bipolar transistorworks in saturation mode (FIG. 28C regime I), in which both E-B and C-Bjunctions are forward biased. The collector current from emitterinjection will be compensated by the forward biased C-B current. So thecollector current goes down with forward C-B voltage. The higher theforward bias on E-B, the higher the forward bias on C-B needed tocompensate the current to zero (FIG. 28C curve 1 to 4).

The n⁺pn bipolar transistors were fabricated by deposition andmachanical manipulation. First, p-type SiNWs were deposited fromsolution onto the substrate. In the second step, the n⁺ and n-type SiNWswere attached to sharp STM tips and released onto the p-type SiNWs underoptical microscope.

The common base current gain of the bipolar transistor in active mode isas large as 0.94 (FIG. 28D) and the common emitter current gain is 16.Three important points are suggested from this large current gain. (a)The efficiency of electron injection from emitter to base is quite high,resulting from the higher doping concentration in emitter than in base.(b) Although the base region is wide (15 μm), the active interactionbetween emitter and collect still exists. Most of injected electronsfrom emitter can go through the base to reach the collector, whichsuggests that the mobility of electrons in base is quite high. (c) Thespace charge region between base and collector has high efficiency tocollect electrons and sweep them into collector, suggesting that theoxide barrier at the interface doesn't contribute significantly, whichfurther confirms our analysis on single pn junctions. Our bipolartransistor can be improved, for example, by reducing the base width, toapproach the performace of the commercial one in which the typicalcommon base current gain is larger than 0.99.

To exploit the applications of these bottom-up building blocks in logiccircuit, and to further demonstrate the capability that contolled dopingof SiNWs can provide us, we create a complementary inverter in form of alightly p and a lightly n-doped SiNW cross. The schematics of an crossedSiNW inverter structure is shown in FIG. 29A (bottom) while that of aninverter in semiconductor physics is shown in FIG. 29A (top). Thelightly doped p and n-type SiNWs in the inverter show very large gateeffect and can be completely depleted as is shown for p-type SiNW inFIG. 29B inset. As seen in FIG. 29B, the output voltage is negative(zero) with the positive (negative) input voltage, which is the typicalinverter behavior. This behavior can explained like this: the depletionof n-type (p-type) wires by negative (positive) input makes the outputequal to ground (bias). The voltage gain is calculated as 0.13, theslope of voltage inversion. The gain is lower than that in commercialinverters which is larger than 1, but can be improved by using thinnergate oxide layer instead of the 600 nm oxide, which reduces the gateresponse of SiNWs, and using more lightly doped SiNWs, which needs moreeffort to make ohmic contact with and to be further investigated.

While two crossed lightly doped p and n-type SiNWs make inverters, twocrossed degenerately doped p⁺ and n⁺-type SiNWs can form tunnel diodes.In contrast to the pn junction, the tunnel diode do not show rectifyingbehavior, but rather show NDR behavior in forward bias, with a PVR of 5to 1 shown in FIG. 29C. The difference can be explained by Esaki diodemechanism. The built-in potential forms when p⁺ and n⁺-type contact eachother, but the space charge region width is thin enough to allowelectron tunneling. Electrons can tunnel through this thin space chargeregion under reverse bias (FIG. 29D left) and low forward bias (FIG. 29Dmiddle) causing the current to flow. Beyond a certain point, a furtherincrease in the forward bias results in the condunction band of then-side moving into the band gap of the p-side (FIG. 29D right) whichsuppresses electron tunneling and thereby reduces current. Furtherincreases of forward bias reduce the built-in potential barrier whichallows thermal excitation mechanism to dominate conduction and thecurrent goes high.

The results described here demonstrate the bottom-up assembly ofmultiple types of nanoscale electronic devices from doped SiNWs withcontrol over both dopant type and doping level. The individual devicesshow predictable behaviors similar to the conventionally fabricateddevices. The mass production and high intergration of these functionalnanodevices can be realized by chemical assembly assisted with electricfield and flowing solution alignment, which will lead to excitingpractical applications in nanoelectronics while avoiding high costfabrication lines. Moreover, we can expect that, in conjunction withoptical signal, pn diode crosses can function as photodiodes and pnsolar cells, and bipolar transistor crosses can form phototransistors.

The alignment of NW by electric field and flowing solution producedone-type of parallel NW arrays. Switching the direction of electricfield and flowing solution to lay down the the other type of NWs canform very dense NW crosses.

FIGS. 27A-27F illustrate crossed SiNW junctions. FIG. 27A shows atypical FE-SEM image of crossed NW junctions with Al/Au as contactleads. The scale bar is 2 μm. The diameters of NWs are in the range of20 to 50 nm. FIGS. 27B-27D show I-V behavior of pn, pp and nn junctions,respectively. The curves 110 and 120 correspond to the I-V behavior ofindividual p and n-type SiNWs in junctions, respectively. The curves 130represent the four-terminal I-V through pn junction in FIG. 27B and twoterminal I-V through pp and nn junction in FIGS. 27C and 27D,respectively. In FIG. 27B, the solid line is I-V by following currentbetween lead 1 and 2 and simultaneouly measuring the voltage betweenlead 3 and 4 while the dashed line correponds to that by followingcurrent between 1 and 4 and measuring voltage between 3 and 2. In FIGS.27C and 27D, the solid lines are I-V across one pair of adjacent leads(1-2) and the dashed lines are those across the other three pairs (1-4,2-3, 3-4). FIGS. 27E and 27F show the energy band diagrams of a pnjunction under forward bias and reverse bias, respectively.

FIGS. 28A-28D illustrate n⁺pn crossed SiNW bipolar transistors. FIG. 28Ashows the common base configuration schematics of an n⁺pn bipolartransistor in semiconductor physics (left) and in crossed SiNW structure(right). The n⁺, p and n-type SiNWs function as emitter, base andcollector, respectively. The base is grounded. The emitter is negativelybiased at specific values. The collector voltage is scanned frompositive to negative. FIG. 28B shows a typical FE-SEM image of SiNWbipolar transistor. The scale bar is 5 μm. FIG. 28C shows a collectorcurrent vs collector-base voltage behavior recorded on an n⁺pntransistor with emitter and base SiNWs 15 um apart. Curve 1 to 4correspond to the behavior at emitter-base voltages of −1, −2, −3, −4V.Regime I and II are separated by dashed line, corresponding tosaturation mode and active mode, respectively. FIG. 28D shows commonbase current gain vs collector-base voltage.

FIGS. 29A-29D illustrate complementary inverters and tunnel diodes. FIG.29A shows schematics of a complementary inverter structure insemiconductor physics (top) and that formed by a lightly doped pn cross(bottom). In bottom schematics, one end of n-type NW is biased at −5Vand one end of p-type NW is grounded. Input voltage is back gate voltageand the other ends of p and n-type NWs are shorted as output terminal.FIG. 29B shows output voltage vs input voltage data in a pn crossinverter. The inset in FIG. 29B is the I-V curves of p-type NW in theinverter. Curve 1 to 5 correspond to I-V at back gate voltage −50, −30,−10, 0 and 10V, respectively. The n-type NW in this inverter has similarI-V behavior and can be completely depleted at a gate voltage of −30V.FIG. 29C shows two terminal measurement data of a tunnel diode made froma heavily doped pn cross. The I-V behavior of individual p and n-typeSiNWs have been tested to be linear. The inset in FIG. 29C spreads outthe part of I-V curve showing NDR. FIG. 29D shows the energy banddiagrams of a crossed SiNW tunnel diode. At reverse bias (e.g. atposition 1 in FIG. 29C), electrons can tunnel through the junction (leftdiagram). At small forward bias (e.g. at position 2 in FIG. 29C),electron tunneling is also permitted (middle diagram). At furtherincreased forward bias (e.g. at position 3 in FIG. 29C), electrontunneling is forbidden (right diagram).

Controlled Placement of Nanowires on Surfaces

-   1. A stable suspension of Nanowires (NWs) in ethanol was prepared by    sonicating NWs in ethanol in a bath sonicator for around 3 minutes.-   2. The substrate (silicon wafer) was covered by a self-assembled    monolayer (SAM) with —NH₂ termination.-   3. The microfluidic molds are made of PDMS. A microchannel formed    when the substrate came in contact with PDMS mold, with three walls    of the conduit corresponding to the molded features in the mold and    the fourth corresponded to the surface of the substrate, which was    chemically modified as described in 2.-   4. The NW suspension flowed through as-made microchannel with an    application of +100 volt bias on the substrate. After a flowing time    around 10 min, the channel was washed with ethanol, then let dry    naturally. When the PDMS stamp was removed, we got NWs arrays    aligned in the flow direction on the substrate surface.-   5. By alteration the flow direction, and applying layer-by-layer    scheme we can get multiple cross-bars out of the NW arrays, which is    supposed to be the most important configuration for the devices we    made from NWs.-   6. By patterning the surface, we can get the NWs aligned    (positioned) in a certain place, thus make it possible to create    more regular arrays of devices.

Patterning process: I. a layer of PMMA was spin-coated on the substratesurface, then use EBL (Electron Beam Lithography) to write pattern, i.e.to selectively exposed Si surface which was later chemicallyfunctionalized. (as in 2). II. Now we have the PMMA trenches, the bottomof which is exposed Si surface covered with —NH₂ SAM. When we flow NWsuspensions over these patterns, (as described in 4, 5, just the surfacein this case is patterned), the NWs will be directed into PMMA trenches.At last we lift off the PMMA, together with the NWs stick on PMMAsurface. So only those stay on the bottom of the PMMA trenches left onthe substrate surface, thus we get clean arrays of devices.

Directed Assembly of One Dimensional Nanostructures Into FunctionalNetworks

One-dimensional nanostructures, such as nanowires and nanotubes,represent the smallest dimension for efficient transport of electronsand excitons, and thus are ideal building blocks for hierarchicalassembly of functional nanoscale electronic and photonic structures. Wereport an approach for the hierarchical assembly of one-dimensionalnanostructures into well-defined functional networks. We show thatnanowires can be assembled into parallel arrays with control of theaverage separation, and by combining fluidic alignment with surfacepatterning techniques that it is also possible to control periodicity.In addition, complex crossed nanowire arrays can be prepared usinglayer-by-layer assembly with different flow directions for sequentialsteps. Transport studies show that the crossed nanowire arrays formelectrically conducting networks, with individually addressable devicefunction at each cross point.

Nanoscale materials, for example, nanoclusters and nanowires (NWs),represent attractive building blocks for hierarchical assembly offunctional nanoscale devices that could overcome fundamental andeconomic limitations of conventional lithography-based fabrication.Research focused on zero-dimensional nanoclusters has led to significantadvances, including the assembly of arrays with order extending fromnanometer to micrometer length scales. In contrast, the assembly ofone-dimensional (1D) nanostructures, such as NWs and carbon nanotubes(NTs), has met with much less success, although these materials offergreat potential as building blocks for applications in nanoelectronicsand photonics.

To achieve the substantial potential of NWs and NTs in these and otherareas of nanotechnology, will require the controlled and predictableassembly of well-ordered structures. We report an approach forhierarchical assembly of 1D nanostructures whereby NWs are aligned influid flows with the separation and spatial location readily controlled.Crossed NW arrays were also prepared using layer-by-layer assembly withdifferent flow directions for sequential steps. Transport studies showthat the crossed NW arrays form electrically conducting networks, withindividually addressable device function at each NW/NW cross point. Thisapproach can be potentially used for organizing other 1D nanostructuresinto highly integrated device arrays, and thus offers a general pathwayfor bottom-up assembly of new electronic and photonic nanosystems.

The gallium phosphide (GaP), indium phosphide (InP) and silicon (Si) NWsused in these studies were synthesized by laser assisted catalyticgrowth, and subsequently suspended in ethanol solution. In general, wehave assembled arrays of NWs by passing suspensions of the NWs throughfluidic channel structures formed between a poly(dimethylsiloxane)(PDMS) mold and a flat substrate (FIGS. 30A and 30B). Parallel andcrossed arrays of NWs can be readily achieved using single (FIG. 30A)and sequential crossed (FIG. 30B) flows, respectively, for the assemblyprocess as described below.

A typical example of parallel assembly of NWs (FIG. 31A) shows thatvirtually all the NWs are aligned along one direction; i.e. the flowdirection. There are also some small deviations with respect to the flowdirection, which we will discuss below. Examination of the assembled NWson larger length scales (FIG. 31B) shows that the alignment readilyextends over hundreds of micrometers. Indeed, alignment of the NWs hasbeen found to extend up to millimeter length scales, and seem to belimited by the size of the fluidic channels, based on experimentscarried out using channels with widths ranging from 50 to 500 μm andlengths from 6-20 mm.

We have carried out several types of experiments to understand factorscontrolling the alignment and average separation of the NWs. First, wefind that the degree of alignment can be controlled by the flow rate.With increasing flow rates, the width of the NW angular distributionwith respect to the flow direction (e.g., inset FIG. 31C) significantlynarrows. Comparison of the distribution widths measured over a range ofconditions shows that the width decreases quickly from our lowest flowrate, ˜4 mm/s, and approaches a nearly constant value at ˜10 mm/s (FIG.31C). At the highest flow rates examined in our studies, more than 80%of the NWs are aligned within ±5 degrees of the flow direction (inset,FIG. 31C). Our observed results can be explained within the framework ofshear flow. Specifically, the channel flow near the substrate surfaceresembles a shear flow and aligns the NWs in the flow direction beforethey are immobilized on the substrate. Higher flow rates produce largershear forces, and hence lead to better alignment.

In addition, the average NW surface coverage can be controlled by theflow duration (FIG. 31D). Experiments carried out at constant flow rateshow that the NW density increases systematically with flow duration. Inthese experiments, a flow duration of 30 min produced a density of ca.250 NWs/100 μm or an average NW/NW separation of ˜400 nm. Extendeddeposition time can produce NW arrays with spacings on the order of 100nm or less. We note that the deposition rate and hence averageseparation versus time depends strongly on the surface chemicalfunctionality. Specifically, we have shown that the GaP, InP and Si NWsdeposit more rapidly on amino-terminated monolayers, which possesses apartial positive charge, than on either methyl-terminated monolayers orbare SiO₂ surfaces. It is also important to recognize that the minimumseparation of aligned NWs that can be achieved without NW-NW contactswill depend on the lengths of the NWs used in the assembly process.Recent progress demonstrating control of NW lengths from the 100nanometer to tens of micrometer scale should increase the range ofaccessible spacings without contact.

Our results demonstrate ordering of NW structure over multiple lengthscales-organization of nanometer diameter wires with 100 nm tomicrometer scale separations over millimeter scale areas. Thishierarchical order can readily bridge the microscopic and macroscopicworlds, although to enable assembly with greatest control requires thatthe spatial position also be defined. We have achieved this importantgoal by utilizing complementary chemical interactions between chemicallypatterned substrates and NWs (FIG. 32A). SEM images of representativeexperiments (FIGS. 32B-32D) show parallel NW arrays with lateral periodsthe same as those of the surface patterns. These data demonstrate thatthe NWs are preferentially assembled at positions defined by thechemical pattern, and moreover, show that the periodic patterns canorganize the NWs into a regular superstructure. It is important torecognize that the patterned surface alone does not provide good controlof the 1D nanostructure organization. Assembly of NTs and NWs onpatterned substrates shows 1D nanostructures aligned with, bridging, andlooping around patterned areas with little directional control. Our useof fluid flows avoids these significant problems and enables controlledassembly in one or more directions. By combining this approach withother surface patterning methods, such as nanoscale domain formation indiblock copolymers and spontaneous ordering of molecules, it should bepossible to generate well-ordered NW arrays beyond the limitations ofconventional lithography.

Our general approach can be used to organize NWs into more complexcrossed structures, which are critical for building dense nanodevicearrays, using the layer-by-layer scheme illustrated in FIG. 31B. Theformation of crossed and more complex structures requires that thenanostructure-substrate interaction is sufficiently strong thatsequential flow steps do not affect preceding ones: we find that thiscondition can be achieved. For example, alternating the flow inorthogonal directions in a two-step assembly process yields crossbarstructures (FIGS. 33A and 33B). Both Figs. show that multiple crossbarscan be obtained with only hundreds of nanometer separations betweenindividual cross points in a very straightforward, low cost, fast andscalable process. Although the separations between individual NWs arenot completely uniform, a periodic array can be easily envisioned usinga patterned surface as described above. Significantly, these crossbarstructures can yield functional devices (see below).

We believe that our approach for directed assembly of multiple crossedNW arrays offers significant advantages over current efforts, which haveused random deposition, direct manipulation of individual NWs and NTsand electric fields to make single crossed structures. With randomdeposition and manipulation it is difficult to obtain multiple crossbarsrequired for integrated nanodevices. While electric fields enable morecontrol over assembly, this method is also limited by (i) electrostaticinterference between nearby electrodes as separations are scaled belowthe micrometer level and (ii) the requirement of extensive lithographyto fabricate the electrodes for assembly of multiple NW devicestructures. Our fluidic approach is intrinsically very parallel andscalable, and moreover, it allows for the directed assembly ofgeometrically complex structures by simply controlling the anglesbetween flow directions in sequential assembly steps. For example, anequilateral triangle (FIG. 33C) was assembled in a three-layerdeposition sequence using 60° angles between the three flow directions.The method of flow alignment thus provides a flexible way to meet therequirements of many device configurations, including those requiringassembly of multiple ‘layers’ of NWs.

Electric fields can be used to align suspensions of semiconductor NWsinto parallel NW arrays and single NW crosses, where patternedmicro-electrode arrays are used to create a field pattern. Fringingfields and charging can, however, lead to significant complications inthe assembly of multiple crosses at the submicron scale. An importantfeature of this layer-by-layer assembly scheme is that each layer isindependent of the others, and thus a variety of homo- and heterojunction configurations can be obtained at each crossed point by simplychanging the composition of the NW suspension used for each step. Forexample, it should be possible to directly assemble and subsequentlyaddress individual nanoscale devices using our approach with n-type andp-type NWs and NTs, in which the NWs/NTs act as both the wiring andactive device elements. A typical 2×2 crossbar array made of n-type InPNWs, in which all eight ends of the NWs are connected by metalelectrodes, demonstrates this point (FIG. 33D). Transport measurements(FIG. 33E) show that current can flow through any two of the eight ends,and enable the electrical characteristics of individual NWs and theNW-NW junctions to be assessed. The current-voltage (I-V) data recordedfor each of the four cross points exhibit linear or nearly linearbehavior (curves 200), and are consistent with expectations for n-n typejunctions. Because single NW/NW p-n junctions formed by randomdeposition exhibit behavior characteristic of light-emitting diodes(LEDs), we believe it is apparent that our approach could be used toassemble high-density and individually addressable nanoLEDs andelectronically more complex nanodevices.

These studies provide a general and rational approach for hierarchicalassembly of 1D nanomaterials into well-defined functional networks thatcan bridge the nanometer through millimeter size regimes. We have shownthat NWs can be assembled into parallel arrays with control of theaverage separation, and by combining fluidic alignment with surfacepatterning techniques that it is also possible to control periodicity.In addition, we have demonstrated the possibility of layer-by-layerassembly of crossed and more complex structures by varying the flowdirection in sequential steps, and have obtained preliminary resultssuggesting that this approach can be extended to 1D nanostructures, suchas carbon NTs. We believe that flow assembly represents a generalstrategy for organization of NW and NT building blocks into structuresneeded for wiring, interconnects and functional devices, and thus couldenable a bottom-up manufacturing paradigm for future nanotechnologies.

Additional studies show that suspensions of single-walled carbonnanotubes and duplex DNA can be aligned in parallel arrays using thefluidic approach.

FIGS. 30A and 30B are schematics of fluidic channel structures for flowassembly. FIG. 30A shows a channel formed when the PDMS mold was broughtin contact with a flat substrate. NW assembly was carried out by flowinga NW suspension inside the channel with a controlled flow rate for a setduration. Parallel arrays of NWs are observed in the flow direction onthe substrate when the PDMS mold is removed. FIG. 30B illustrates thatmultiple crossed NW arrays can be obtained by changing the flowdirection sequentially in a layer-by-layer assembly process.

FIGS. 31A-31D illustrate parallel assembly of NW arrays. FIGS. 31A and31B are SEM images of parallel arrays of InP NWs aligned in channelflow. The scale bars correspond to 2 μm and 50 μm in FIGS. 31A and 31B,respectively. The silicon (SiO₂/Si) substrate used in flow assembly wasfunctionalized with an amino-terminated self assembled monolayer (SAM)by immersion in a 1 mM chloroform solution of3-aminopropyltriethoxysilane (APTES) for 30 min, followed by heating at110° C. for 10 min. All of the substrates used in the followingexperiment were functionalized in a similar way unless otherwisespecified. FIG. 31C shows NW angular spread with respect to the flowdirection vs. flow rate. Each data point in the Fig. was obtained bystatistical analysis of angular distribution of ˜200 NWs (e.g., seeinset). The inset shows histogram of NW angular distribution at a flowrate of 9.40 mm/s. FIG. 31D shows the average density of NW arrays vs.flow time. The average density was calculated by dividing the averagenumber of NWs at any cross section of the channel by the width of thechannel. All of the experiments were carried out with a flow rate of6.40 mm/s.

FIGS. 32A-32D illustrate assembly of periodic NW arrays. FIG. 32A is aschematic view of the assembly of NWs onto a chemically patternedsubstrate. The light gray areas correspond to amino-terminated surfaces,while the dark gray area corresponds to either methyl-terminated or baresurfaces. NWs are preferentially attracted to the amino-terminatedregions of the surface. FIGS. 32B and 32C show parallel arrays of GaPNWs aligned on poly(methylmethacrylate) (PMMA) patterned surface with 5μm and 2 μm separation. The dark regions in the image correspond toresidual PMMA, while the bright regions correspond to theamino-terminated SiO₂/Si surface. The NWs are preferentially attractedto amino-terminated regions. The PMMA was patterned with standardelectron beam (E-beam) lithography, and the resulting SiO₂ surface wasfunctionalized by immersing in a solution of 0.5% APTES in ethanol for10 min, followed by 10 min at 100° C. The scale bars correspond to 5 μmand 2 μm in FIGS. 32B and 32C, respectively. FIG. 32D shows parallelarrays of GaP NWs with 500 nm separation obtained using a patterned SAMsurface. The SiO₂/Si surface was first functionalized withmethyl-terminated SAM by immersing in pure hexamethyldisilazane (HMDS)for 15 min at 50° C., followed by 10 min at 110° C. This surface waspatterned by E-beam lithography to form an array of parallel featureswith 500 nm period, followed by functionalization using APTES. The scalebar corresponds to 500 nm.

FIGS. 33A-33E illustrate layer-by-layer assembly and transportmeasurements of crossed NW arrays. FIGS. 33A and 33B show typical SEMimages of crossed arrays of InP NWs obtained in a two-step assemblyprocess with orthogonal flow directions for the sequential steps. Flowdirections are highlighted by arrows in the images. FIG. 33C shows anequilateral triangle of GaP NWs obtained in three-step assembly process,with 60° angles between flow directions, which are indicated by numberedarrows. The scale bars correspond to 500 nm in the three images. FIG.33D shows an SEM image of a typical 2×2 cross array made by sequentialassembly of n-type InP NWs using orthogonal flows. Ni/In/Au contactelectrodes, which were deposited by thermal evaporation, were patternedby E-beam lithography. The NWs were briefly (3-5 s) etched in 6% HFsolution to remove the amorphous oxide outer layer prior to electrodedeposition. The scale bar corresponds to 2 μm. FIG. 33E showsrepresentative I-V curves from two-terminal measurements on a 2×2crossed array. The curves 210 represent the I-V of four individual NWs(ad, bg, cf, eh), and the curves 200 represent I-V across the four n-ncrossed junctions (ab, cd, ef, gh).

We have demonstrated field effect transistors, pn junctions, lightemission diodes, bipolar transistors, complementary inverters, tunneldiodes. We can make all the existing types of semiconductor devicesusing nanowires. The following are potential applications:

(1) Chemical and biological sensors

(2) Memory and computing

(3) Photodetector and polarized light detector

(4) Indicating tag using the photoluminescence properties

(5) Single electron transistors

(6) Lasers

(7) Photovoltaic solar cells

(8) Ultra-sharp tip for scanning probe microscopy and near-filed imaging

(9) Ultra-small electrodes for electrochemical and biologicalapplications

(10) Interconnect wires for nanoelectronics and optoelectronics

(11) Temperature sensors

(12) Pressure sensors

(13) Flow sensors

(14) Mass sensors

(15) Single photon emitters and detectors

(16) Ballistic transport and coherent transport for quantum computing

(17) Spintronics devices

(18) Assembly of nanowires for 2D and 3D photonic bandgap materials

The following is a description of alternate techniques for assemblingnanowires to form devices. Fluidics can be used to assemble nanowires.

Nanowires (or any other elongated structures) can be aligned by inducinga flow of nanowire solution on surface, wherein the flow can be achannel flow or flow by any other ways.

Nanowire arrays with controlled position and periodicity can be producedby patterning the surface of the substrate and/or conditioning surfaceof the nanowires with different functionalities.

Wherein the position and periodicity control is achieved by designingspecific complementary forces (chemical or biological or electrostaticor magnetic or optical) between the patterned surface and wires, such asA wire goes to A′ patterned area, B wire goes to B' patterned area, Cwire goes to C′ patterned area, etc.

Wherein the surface of the substrate and/or nanowires can be conditionedwith different molecules/materials, or different charges, differentmagnetos or different light intensities (e.g., byinterference/diffraction patterns from light beams) or a combination ofthese.

As-assembled nanowire arrays could also be transferred to anothersubstrate (e.g. by stamping).

Nanowires can be assembled by complementary interaction. Flow is usedfor assembly of nanowires in the above methods, although it is notlimited to flow only. Complementary chemical, biological, electrostatic,magnetic or optical interactions alone can also be exploited fornanowire assembly (although with less control).

Nanowires can be assembled using physical patterns. Deposit nanowiresolution onto substrate with physical patterns, such as surface steps,trenches, etc.

Nanowires can be aligned along the corner of the surface steps or alongthe trenches.

Physical patterns can be formed by the natural crystal lattice steps orself- assembled diblock copolymer stripes, or imprinted patterns or anyother patterns.

Nanowires may be assembled by electrostatic or magnetic force betweennanowires. By introducing charge onto nanowire surface, electrostaticforces between nanowires can align them into certain patterns, such asparallel arrays.

Nanowires can be assembled using a LB film. Nanowires were first surfaceconditioned and dispersed to the surface of a liquid phase to form aLangmuir-Blodgett

(LB) film. Nanowires can then be aligned into different patterns (suchas parallel arrays) by compressing the surface. Then the nanowirepatterns can be transferred onto desired substrate.

Nanowires can be assembled by shear stretching by dispersing nanowiresin a flexible matrix (which could be polymers), followed by stretchingthe matrix in one direction, nanowires can be aligned in the stretchingdirection by the shear force induced. The matrix can then be removed andthe aligned nanowire arrays can be transferred to desired substrate.

Wherein the stretching of the matrix can be induced by mechanical,electrical optical, magnetic force. And the stretching direction couldbe either in the plane of the substrate or not.

Having now described some illustrative embodiments of the inventionclaimed below, it should be apparent to those skilled in the art thatthe foregoing is merely illustrative and not limiting, having beenpresented by way of example only. Numerous modification and otherillustrative embodiments are within the scope of one of ordinary skillin the art and are contemplated as falling within the scope of theclaims set forth below. In particular, although many of the examplespresented herein involve specific combinations of method acts or systemelements, it should be understood that those acts and those elements maybe combined in other ways to accomplish the same objectives. Acts,elements and features discussed only in connection with one embodimentof a system or method are not intended to be excluded from a similarrole in other embodiments. Further, for the one or moremeans-plus-function limitations recited in the following claims, themeans are not intended to be limited to the means disclosed herein forperforming the recited function, but are intended to cover in scope anyequivalent means, known now or later developed, for performing therecited function.

1. A method of assembling one or more nanoscale elongated structures ona surface, the method comprising acts of: (A) flowing a liquid thatcomprises the one or more nanoscale elongated structures onto thesurface which nanoscale elongated structures have at least one portionhaving a smallest dimension of less than 500 nm; and (B) aligning theone or more nanoscale elongated structures on the surface to form anarray of the nanoscale elongated structures. (C) conditioning thesurface with one or more functionalities that attract the one or morenanoscale elongated structures to particular positions on the surface,wherein act (B) comprises attracting the one or more nanoscale elongatedstructures to the particular positions using the one or morefunctionalities.
 2. The method of claim 1, wherein act (C) comprises:conditioning the surface with one or more molecules.
 3. The method ofclaim 1, wherein act (C) comprises: conditioning the surface with one ormore charges.
 4. The method of claim 1, wherein act (C) comprises:conditioning the surface with one or more magnetos.
 5. The method ofclaim 1, wherein act (C) comprises: conditioning the surface with one ormore light intensities.
 6. The method of claim 1, wherein act (C)comprises: conditioning the surface with one or more functionalitiesthat attract the one or more nanoscale elongated structures toparticular positions on the surface using chemical force.
 7. The methodof claim 1, wherein act (C) comprises: conditioning the surface with oneor more functionalities that attract the one or more nanoscale elongatedstructures to particular positions on the surface using optical force.8. The method of claim 1, wherein act (C) comprises: conditioning thesurface with one or more functionalities that attract the one or morenanoscale elongated structures to particular positions on the surfaceusing electrostatic force.
 9. The method of claim 1, wherein act (C)comprises: conditioning the surface with one or more functionalitiesthat attract the one or more nanoscale elongated structures toparticular positions on the surface using magnetic force.
 10. The methodof claim 1, wherein the one or more nanoscale elongated structurescomprises a nanowire.
 11. The method of claim 1, wherein the one or morenanoscale elongated structures comprises a semiconductor nanowire. 12.The method of claim 1, wherein the one or more nanoscale elongatedstructures comprises a bulk-doped semiconductor nanowire.
 13. The methodof claim 1, wherein the one or more nanoscale elongated structures hasat least one portion having a smallest width of less than 500nanometers.
 14. The method of claim 11, wherein the semiconductornanowire is doped during growth of the nanowire.
 15. The method of claim11, wherein the semiconductor nanowire is grown catalytically from acatalyst particle.
 16. The method of claim 11, wherein the semiconductornanowire is grown using laser-assisted catalytic growth.
 17. The methodof claim 1, wherein said aligning comprises generating an electric fieldbetween at least two electrodes, and positioning the one or morenanoscale elongated structures between the electrodes.
 18. A method ofassembling one or more nanoscale elongated structures on a surface, themethod comprising acts of: (A) flowing a liquid that comprises the oneor more nanoscale elongated structures onto the surface which nanoscaleelongated structures have at least one portion having a smallestdimension of less than 500 nm; and (B) aligning the one or morenanoscale elongated structures on the surface to form an array of thenanoscale elongated structures; and (C) patterning the surface toreceive the one or more nanoscale elongated structures at particularpositions on the surface.
 19. The method of claim 18, wherein act (C)comprises: creating physical patterns on the surface.
 20. The method ofclaim 19, wherein the physical patterns are trenches.
 21. The method ofclaim 19, wherein the physical patterns are steps.
 22. The method ofclaim 19, wherein the surface is a surface of a substrate, and whereincreating physical patterns on the surface comprises: using crystallattice steps of the substrate.
 23. The method of claim 19, wherein thesurface is a surface of a substrate, and wherein creating physicalpatterns on the surface comprises: using self-assembled di-block polymerstrips.
 24. The method of claim 19, wherein creating physical patternson the surface comprises: using patterns.
 25. The method of claim 24,wherein creating physical patterns on the surface comprises: usingimprinted patterns.
 26. The method of claim 18, wherein the one or morenanoscale elongated structures comprises a nanowire.
 27. The method ofclaim 18, wherein the one or more nanoscale elongated structurescomprises a semiconductor nanowire.
 28. The method of claim 18, whereinthe one or more nanoscale elongated structures comprises a bulk-dopedsemiconductor nanowire.
 29. The method of claim 18, wherein the one ormore nanoscale elogated structures has at least one portion having asmallest width of less than 500 nanometers.
 30. The method of claim 27,wherein the semiconductor nanowire is doped during growth of thenanowire.
 31. The method of claim 27, wherein the semiconductor nanowireis grown catalytically from a catalyst particle.
 32. The method of claim27, wherein the semiconductor nanowire is grown using laser-assistedcatalytic growth.
 33. The method of claim 18, wherein said aligningcomprises generating an electric field between at least two electrodes,and positioning the one or more elongated structures between theelectrodes.
 34. A method of assembling one or more elongated structureson a surface, wherein one or more of the elongated structures arecontained in a liquid and the one or more of the elongated structuresare at least one of the following: a single crystal, an elongatedsemiconductor that, at any point along its longitudinal axis, has alargest cross-sectional dimension less than 500 nanometers, and afree-standing semiconductor with at least one portion having a smallestwidth of less than 500 nanometers, and wherein the method comprises actsof: (A) conditioning the surface with one or more functionalities thatattract the one or more elongated structures to particular positions onthe surface, and (B) aligning the one or more elongated structures byattracting the one or more elongated structures to the particularpositions using the one or more functionalities.
 35. The method of claim34, wherein act (A) comprises: conditioning the surface with one or moremolecules.
 36. The method of claim 34, wherein act (A) comprises:conditioning the surface with one or more charges.
 37. The method ofclaim 34, wherein act (A) comprises: conditioning the surface with oneor more magnetos.
 38. The method of claim 34, wherein act (A) comprises:conditioning the surface with one or more light intensities.
 39. Themethod of claim 34, wherein act (A) comprises: conditioning the surfacewith one or more functionalities that attract the one or more elongatedstructures to particular positions on the surface using chemical force.40. The method of claim 34, wherein act (A) comprises: conditioning thesurface with one or more functionalities that attract the one or moreelongated structures to particular positions on the surface usingoptical force.
 41. The method of claim 34, wherein act (A) comprises:conditioning the surface with one or more functionalities that attractthe one or more elongated structures to particular positions on thesurface using electrostatic force.
 42. The method of claim 34, whereinact (A) comprises: conditioning the surface with one or morefunctionalities that attract the one or more elongated structures toparticular positions on the surface using magnetic force.
 43. The methodof claim 34, wherein the one or more elongated structures comprises ananowire.
 44. The method of claim 34, wherein the one or more elongatedstructures comprises a semiconductor nanowire.
 45. The method of claim34, wherein the one or more elongated structures comprises a bulk-dopedsemiconductor nanowire.
 46. The method of claim 34, wherein the one ormore elogated structures has at least one portion having a smallestwidth of less than 500 nanometers.
 47. The method of claim 44, whereinthe semiconductor nanowire is doped during growth of the nanowire. 48.The method of claim 44, wherein the semiconductor nanowire is growncatalytically from a catalyst particle.
 49. The method of claim 44,wherein the semiconductor nanowire is grown using laser-assistedcatalytic growth.
 50. The method of claim 34, wherein said aligningcomprises generating an electric field between at least two electrodes,and positioning the one or more elongated structures between theelectrodes.